INTRODUCTION

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The development of vaccines for prevention of infectious diseases has revolutionized our approach to public health. In many countries people enjoy better health because of effective immunization programs which have diminished the morbidity and mortality of once common infectious diseases (Table 1). In a recent compilation of the most important public health advances of the 20th century by the Centers for Disease Control and Prevention (CDC), immunizations were ranked first (107). The success of immunizations is illustrated by the 1977 eradication of smallpox after a 10-year effort directed by the World Health Organization (WHO) and the extraordinary progress toward the global elimination of poliomyelitis in the 1990s (99, 375).

To achieve this progress in public health, scientists and physicians have combined efforts to understand the biology of infectious agents. They have worked to purify the agents and, in some cases, their components; to develop and test vaccines; and to manufacture and administer these vaccines to appropriate segments of the population. In addition, government and physician advisory groups have developed appropriate recommendations and schedules for immunization.

The purpose of this article is to review the changes that have taken place in active immunization in the United States over the past decade. Since 1990, new and improved vaccines have become available for ten diseases and four new combination vaccines have been developed (Table 2). New or improved vaccines are under development for three additional diseases. In addition, immunization strategies for four diseases have changed and investigational vaccines have been developed for a broad array of infections.

DISEASES FOR WHICH NEW VACCINES BECAME AVAILABLE

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Hepatitis A

Hepatitis A virus (HAV) infection is most prevalent in developing countries, reflecting the primary route of transmission of fecal-oral, person-to-person spread. HAV also remains the most frequent cause of acute viral hepatitis in the United States and has led to substantial morbidity and associated costs (93, 237). The incidence of disease varies considerably among different populations in the United States (93, 237, 340). Community-wide outbreaks recurring every 3 to 10 years in high-risk communities account for much of disease occurrence and are a primary target for control by vaccination. Rates of infection are highest among Alaskan Natives and American Indians. Other groups at increased risk include travelers to developing countries, homosexual and bisexual men, and users of illicit drugs. However, 45% of reported cases have no identifiable risk factor indicating that selective immunization is unlikely to have a major impact on the control of HAV.

Outbreaks among children attending day care centers and their staff are common and have been associated with community outbreaks (197). However, the prevalence of HAV infection in day care center staff and among children and adolescents who previously attended day care is not increased, suggesting that infections within day care settings most commonly reflect transmission within the community (93, 214).

Disease in the United States is most common in children 5 to 14 years of age (93). Infection rates are appreciable in younger children in whom infection is usually asymptomatic and who serve as a silent reservoir. Children and infants can shed HAV for longer periods than adults, up to several months after onset of clinical illness. Because children frequently have asymptomatic infections and may shed virus for prolonged periods, they play an important role in HAV transmission. In one study in adults without an identified source of infection, 52% of their households included a child younger than 6 years (94). Thus, control of HAV by active immunization will likely necessitate universal childhood immunization.

Both inactivated and attenuated HAV vaccines have been developed (157). However, only inactivated vaccines have been licensed in the United States. Inactivated HAV vaccine is prepared by methods similar to those used for inactivated poliomyelitis vaccine. Virus is propagated in human diploid fibroblast cell cultures, formalin inactivated, and adsorbed to aluminum hydroxide adjuvant (18, 93). Two such products have been licensed in the United States, HAVRIX (SmithKline Beecham Biologicals) in 1995 and VAQTA (Merck & Co.) in 1996. Each vaccine has two formulations, an adult and a pediatric product of different antigen content. The pediatric formulation is indicated for persons 2 to 18 years of age. The vaccines can be used interchangeably (57).

Inactivated HAV vaccine is highly immunogenic. After a single dose, 95% of children and nearly all adults seroconvert within 1 month (18, 93). Following a second dose in children, seroconversion approaches 100%.

Concurrent administration of immune globulin and vaccine inhibits the peak serum antibody concentration achieved but not the rate of seroconversion (363). Since the antibody concentrations are well above the protective concentration, this inhibition is not considered to be clinically significant and supports passive-active immunoprophylaxis when indicated.

In two large clinical trials of inactivated HAV vaccine in children older than 2 years, protective efficacy has been greater than 90% (207, 371). In a double-blind, placebo-controlled, randomized study in Thailand involving approximately 34,000 vaccinees, the protective efficacy against clinical hepatitis A was 94% following two doses given 1 month apart; it was 100% following a subsequent 12-month booster dose (207). Some early studies suggested that passively acquired maternal HAV antibody might interfere with vaccine immunogenicity in infants. Subsequent studies have been conflicting (223, 296). Until this issue is fully resolved, the use of HAV vaccine in children younger than 2 years is not recommended.

Vaccination is also effective in controlling outbreaks in communities with a high rate of disease (93). For example, in a New York state community in which hepatitis A is highly endemic in children, a single dose of vaccine was 100% effective beginning 3 weeks after immunization in preventing symptomatic disease. (371).

Data are not available to determine if and when booster doses would be indicated. However, the duration of protection following vaccination is likely to be prolonged. While the data on persistence of serum antibody and protection against infection are limited to approximately 5 years of experience, adults have been demonstrated to maintain protective antibody concentrations for at least 6 years and kinetic models of antibody decline indicate possible antibody persistence for 20 years (360).

Except for rare reports of anaphylaxis and anaphylactoid reaction in adults in Europe and Asia, serious reactions to inactivated HAV vaccine have not been reported (93). Pain, tenderness, and induration at the injection site can occur (207).

HAV vaccine is currently recommended only for persons 2 years of age or older who are at increased risk of infection (18, 94). The indications are as follows: persons traveling to or working in a country with high or intermediate incidence of HAV infection; persons with clotting factor disorders; sexually active homosexual and bisexual males; illicit drug users; and persons working with infected primates or with HAV in a laboratory. Routine vaccination of persons with chronic liver disease including other forms of infectious hepatitis is recommended since they may be at increased risk of fulminant hepatitis if they become infected with HAV. Children living in areas where rates of hepatitis A are at least twice the national average should be routinely vaccinated. In addition, children living in areas where rates of hepatitis A are at least greater than the national average but lower than twice the national average should be considered for routine vaccination (94). Although the CDC Advisory Committee on Immunization Practices (ACIP) guidelines do not recommend routine vaccination of food handlers, consideration may be given to vaccination of these workers in areas where state and local health authorities or private employers determine that vaccine is cost-effective (94).

Vaccination should also be considered for control of hepatitis A outbreaks in communities in which the rate of infection is increased. Production of a highly immune population reduces the incidence of hepatitis A and decreases transmission by preventing fecal shedding of HAV (94). However, since effectiveness of vaccination has not been demonstrated in localized outbreaks occurring in institutions for the developmentally disabled, day care centers, schools and prisons, intramuscular immune globulin currently is recommended for close contacts of infected persons in these circumstances. At present, HAV vaccine is not routinely indicated for day care center attendees and staff or for food handlers.

To control the significant public health burden of hepatitis A in the future, licensure of HAV vaccine for infants and development of combination products containing HAV and other vaccine antigens may lead to inclusion of HAV vaccine in the routine childhood immunization program.

Japanese encephalitis virus, the most important cause of epidemic arbovirus encephalitis in Asia, has a wide clinical spectrum ranging from asymptomatic infection to severe infection with permanent neurologic sequelae and a high case fatality rate of 30 to 70% (272, 358). Despite the high fatality rate, vaccine has not been available in the United States until recently. In December 1992, a JE virus vaccine was licensed in the United States for use in persons living in or traveling to Asia.

JE vaccine is a formalin-inactivated virus derived from purified infected mouse brain (80). Immunogenicity studies in the United States indicate that three doses are needed to provide protective concentrations of antibody in more than 80% of vaccinees (305). The longevity of neutralizing antibody after the primary vaccination series is not known. In one Japanese study, protective antibody titers persisted for 3 years after a booster dose (236).

A field trial of the currently licensed JE vaccine which was conducted in Thai children demonstrated an efficacy of 91% compared with placebo (203). Efficacy was 80% for a single year for a prototype of the currently licensed vaccine field tested in Taiwanese children (205).

Local reactions to JE vaccine occur in about 20% of vaccinees, while approximately 10% have reported systemic side effects such as fever, headache, malaise, or rash (80). Hypersensitivity type reactions characterized by urticaria and/or angioedema of the extremities, face, and oropharynx have been reported. The median interval between the first dose of vaccine and onset of symptoms is 12 h. After a second dose the interval is generally longer, with a median of 3 days; it may be as long as 2 weeks. Reactions have occurred after a second or third dose when preceding doses did not cause symptoms. The reaction rates are similar after both first and second doses and are approximately 15 to 62 per 10,000 immunizations among U.S. citizens. Neurologic adverse reactions, including acute disseminated encephalomyelitis have also been reported. In Denmark, acute disseminated encephalomyelitis has been estimated to occur in 1 in 50,000 to 75,000 vaccinees. However, a recent review of postmarketing data in the United States from 1993 to 1999 found no serious neurologic events after JE immunization (349).

The JE vaccine is recommended for persons who will be residing in areas where JE is endemic or epidemic (80). The risk for acquiring JE is highly variable within regions of endemic infection. Therefore, the incidence of JE in the area of residence, conditions of housing, nature of activities, and the possibility of unexpected travel to high-risk areas are factors that should be considered in the decision to vaccinate. JE vaccine is not recommended for all travelers to Asia. The vaccine should be offered to persons spending 1 month or longer in areas of endemic infection during the transmission season, especially if travel will include rural areas. The yellow book, Health Information for International Travel, updated regularly by the CDC, provides a useful table that lists affected areas by country and also notes the transmission season.

The recommended primary immunization series is three doses administered on days 0, 7, and 30. An abbreviated schedule of days 0, 7, and 14 can be used when a longer schedule is impractical because of time constraints. Although two doses administered 1 week apart will confer short-term immunity in 80% of vaccinees, this schedule should be used only under unusual circumstances. The last dose should be administered at least 10 days before travel begins to ensure an adequate immune response and access to care if a delayed adverse reaction occurs (80). No data are available on vaccine safety and efficacy in infants (4). The duration of protection is unknown, and definitive recommendations cannot be given on the timing of booster doses. Booster doses may be administered after 2 years.

Since generalized urticaria and angioedema can occur within minutes to as long as 2 weeks after vaccination, medications including epinephrine and equipment to treat anaphylaxis should be available. Vaccinees should be observed for 30 min following vaccination and should be warned about the possibility of delayed urticaria and angioedema. Vaccinees should be advised to remain in areas with ready access to medical care for 10 days after receiving a dose of JE vaccine.

Lyme disease, caused by the spirochete Borrelia burgdorferi, is the most common tick-borne infection in the United States. More than 90% of cases have been reported from the northeastern and north central states. In 1998, 16,802 cases were reported to the CDC, a 70% increase from the 9,896 cases in 1992 (287). Persons of all ages are susceptible to infection, but the highest reported rates of Lyme disease occur in children 5 to 9 years of age and adults aged 45 to 54 years. Transmission peaks from April through July, when the nymphal stages of the tick vectors of Lyme disease, Ixodes scapularis and I. pacificus, are actively seeking hosts. These ticks are found primarily in leaf litter and low-lying vegetation in wooded, brushy, or overgrown grassy areas.

An estimated 85% of persons with symptomatic Lyme disease have the characteristic rash, erythema migrans (279). Untreated infection can cause arthritis or neurologic symptoms, such as radiculoneuropathy or encephalopathy. At any stage, the disease can usually be successfully treated with standard antibiotic regimens.

Vaccines to prevent Lyme disease have been recently developed which contain an immunogenic recombinant protein of B. burgdorferi, outer surface protein A (OspA), which has been lipidated for optimal immunity. One of these vaccines, LYMErix (SmithKline Beecham Biologicals), an aluminum-adjuvanted OspA vaccine, was licensed in 1998. OspA is not expressed by B. burgdorferi in the mammalian host. Immunization with OspA vaccine stimulates the production of antibodies specific for OspA. When a tick takes a blood meal from a vaccinated individual, it ingests these antibodies, which then bind to the surface of B. burgdorferi present in the tick midgut. As a result, B. burgdorferi is prevented from migrating to the tick salivary glands, where it can be transferred to the human host and cause disease. Thus, the OspA vaccine is considered to be a transmission-blocking vaccine. Because of the unique mechanism of action of the OspA vaccine, it is likely that high levels of circulating antibody will have to be maintained in vaccinees to prevent against infection at the time of a bite from an infected tick.

The licensed OspA vaccine was tested in a multicenter, double-blind, placebo-controlled clinical trial which involved 10,936 subjects, aged 15 to 70 years, from regions of the United States where Lyme disease is endemic. Volunteers were given three injections of OspA vaccine or placebo at 0, 1, and 12 months. Vaccine efficacy against definite infection was demonstrated in 76% of those given three injections and 49% of those who received two doses of vaccine. Efficacy against asymptomatic infection was 83% after two doses and 100% after three doses (339).

Although antibody to OspA has been demonstrated to provide a first line of defense by blocking transmission, many believe that the addition of other borrelial antigens, especially those that play a major role in virulence and/or the pathogenesis of this infection, may be needed to increase the efficacy of a vaccine against Lyme disease. Several preclinical studies are in progress to identify and characterize such virulence antigens and evaluate their potential use in candidate vaccines.

The licensed Lyme disease vaccine (LYMErix) is indicated for use in persons aged 15 to 70 years (19, 100). Three doses are administered by intramuscular injection. The initial dose is followed by a second dose 1 month later and a third dose 12 months after the first. Vaccine administration should be timed so the the second dose and the third dose are given several weeks before the beginning of the B. burgdorferi transmission season, which usually begins in April (100). The duration of immunity following the three-dose vaccination series is unknown, and the need for booster doses has not been determined. Accelerated schedules of administration and use of vaccine in persons younger than 15 years are under study.

Local reactions at the site of injection were reported by significantly more vaccinees (24%) than placebo recipients (7.6%) (339). Reports of myalgia, influenza-like illness, fever, and chills within 30 days after a dose were significantly more common among vaccinees than placebo recipients, but none of these were reported by greater than 5% of either group (339). Reports of arthritis were not significantly different between vaccinees and placebo recipients, but vaccine recipients reported significantly more transient arthralgia and myalgia following each dose of vaccine (339). There is a serious theoretical concern that OspA vaccine might induce chronic inflammatory arthritis in genetically susceptible individuals through molecular mimicry of a human lymphocyte surface antigen, lymphocyte function antigen 1 (hLFA-1) (192). Ongoing postmarketing surveillance has not demonstrated any cause for concern thus far.

Recommendations for use of Lyme disease vaccine developed by ACIP were issued in June 1999 (100). Lyme disease vaccine should be targeted to persons at risk for exposure to infected ticks. This risk can be assessed by considering whether a person resides in an area where Lyme disease is highly endemic and the extent to which a person's activities place him or her in contact with ticks (154). Vaccine should not be administered to individuals with treatment-resistant Lyme arthritis.

Vaccination of persons with frequent or prolonged exposure to ticks in areas where Lyme disease is endemic is likely to be an important preventive strategy. For persons with only brief or intermittent exposure to tick habitats in areas where Lyme disease is endemic, the public health benefits of vaccination, compared with early diagnosis and treatment of Lyme disease, are not clear. Vaccination should not be considered a substitute for other preventive measures, such as avoiding tick habitats, wearing protective clothing, using repellents to avoid tick attachment, and promptly removing attached ticks, since the vaccine is less than 100% efficacious and does not provide protection against other tick-borne illnesses.

Serological correlates of immunity against rotavirus infection have not been established. As a result, field studies have been necessary to demonstrate the efficacy of rotavirus vaccines. The efficacy of RRV-TV was evaluated in four field trials, two in the United States (311, 323) and one each in Venezuela (294) and Finland (219). The findings of all four studies were similar; the vaccine demonstrated efficacies of 48 to 68% against any rotavirus diarrhea, 38 to 91% against moderate disease, and 70 to 100% against severe diarrhea. The studies demonstrated a 50 to 100% efficacy in preventing doctor visits for evaluation and treatment of rotavirus diarrhea. The vaccine was also effective in reducing the duration of rotavirus diarrhea. The trial in Finland was large enough to examine the efficacy of the vaccine in preventing rotavirus hospitalizations: protection was 100% (219). In this study, vaccinated children were also protected from nosocomially acquired rotavirus diarrhea. Extended follow-up in the study in Finland demonstrated that protection against severe disease persisted through three rotavirus seasons (220). No data are available on the efficacy of administration of fewer than three doses of RRV-TV.

Because infections with serotype G1 viruses have predominated in most studies, the efficacy of RRV-TV against this serotype is well established. In studies conducted in the United States and Finland, RRV-TV was also effective in preventing nonserotype G1 disease (219, 311, 323).

In the clinical trials, RRV-TV was administered to almost 7,000 infants aged 6 to 28 weeks. Following the first dose, there was a statistically significant excess of both low-grade fever (>38°C) and high fever (>39°C) compared with placebo recipients. Fevers generally occurred 3 to 5 days after administration of vaccine, were low grade, and were seen in fewer than 25% of recipients. Decreased appetite, irritability, and decreased activity also were reported following the first dose of vaccine in some trials; these symptoms were highly associated with the presence of fever (221). A statistically significant excess of fever of >38°C was also noted after the second dose of RRV-TV; no increase in any symptoms was noted after the third dose of RRV-TV. In the efficacy study in Finland (221), vaccinated children had a significantly increased rate of diarrhea after the first dose of vaccine compared with placebo recipients; diarrhea was also associated with the presence of fever (221). No significant differences in vomiting were demonstrated between vaccinees and placebo recipients.

In clinical trials, intussusception was noted among 5 of approximately 10,000 vaccine recipients. This number was not significantly higher than among placebo recipients in the studies (312). However, the ACIP, in its recommendation for routine rotavirus vaccination of infants, indicated that postlicensure surveillance was needed for intussusception and any other rare adverse events that might occur following receipt of the vaccine.

Routine immunization with three doses of RRV-TV was recommended for infants at ages 2, 4, and 6 months (21, 105). An estimated 1.5 million doses of rotavirus vaccine were administered to infants from September 1998 until July 1999. In July 1999, the CDC recommended that health care providers and parents postpone use of RRV-TV for infants based on reports to the Vaccine Adverse Event Reporting System (VAERS) of intussusception in 15 infants who received rotavirus vaccine (81). Also at that time, the manufacturer, in consultation with the Food and Drug Administration (FDA), voluntarily ceased further distribution of the vaccine.

In response to the VAERS reports, a preliminary analysis of data from an ongoing postlicensure study at Northern California Kaiser Permanente (NCKP) was performed (81). The rate of intussusception among never-vaccinated children was 45 per 100,000 infant-years, and the rate among children who had received RRV-TV was 125 per 100,000 infant-years. The rate was increased among children who had received RRV-TV during the preceding 3 weeks (219 per 100,000 infant-years) and among children who had received RRV-TV during the previous week (314 per 100,000 infant-years). In addition, preliminary data on intussusception from Minnesota were analyzed (81). The observed rate of intussusception within 1 week of receipt of RRV-TV was 292 per 100,000 infant-years. The preliminary data from Minnesota and from NCKP both suggested an increased risk for intussusception following receipt of RRV-TV. However, the number of cases of intussusception among vaccinated children was small both at NCKP and in Minnesota, and neither study had adequate power to establish a statistically significant difference in the incidence of intussusception among vaccinated and unvaccinated children.

Several studies were undertaken to further define and quantify the association. A multistate case-control study of cases of intussusception occurring between 1 November 1998 and 30, June 1999 in the 19 states with the highest reported vaccine distribution was begun in June 1999. Results of this case-control study show a significantly elevated risk during the 3 to 14 days following vaccination, with an estimated attributable risk of 1 case of intussusception for every 4,670 to 9,474 infants vaccinated (277). A managed-care organization cohort study in 10 health maintenance organizations with automated databases for case finding and vaccine status found that the incidence rate of intussusception was 25/100,000 person-years among unexposed infants and 340/100,000 person-years 3 to 7 days postvaccination. The attributable risk was one case of intussusception per 11,073 children vaccinated (239).

On 22, October 1999, the ACIP, after a review of scientific data from several sources, including preliminary data from both the managed-care cohort and the multistate case-control studies, concluded that intussusception occurs with significantly increased frequency in the first 1 to 2 weeks after vaccination with RRV-TV, particularly following the first dose (116). The ACIP and the American Academy of Pediatrics (AAP) withdrew their recommendations for vaccination of infants in the United States with RRV-TV. The manufacturer has subsequently ceased marketing the vaccine.

The relation between intussusception and RRV-TV is not understood and merits further research. The findings could impact directly on the use of this and other rotavirus vaccines. In addition, the worldwide burden of rotavirus disease remains substantial. Thus, the ACIP decision may not be applicable to other settings, where the burden of disease is substantially higher and where the risks and benefits of rotavirus vaccination could be different.

Varicella is currently the most common childhood infectious disease in the United States. Before the availability of varicella vaccine, varicella infection was responsible for an estimated 4 million cases, 11,000 hospitalizations, and 100 deaths each year in the United States (97). Approximately 90% of cases occurred in children, with the highest incidence among children aged 1 to 6 years. In recent years, severe infections with group A beta-hemolytic streptococci have complicated varicella, leading to considerable morbidity and mortality in otherwise healthy individuals (115).

Varicella vaccine was licensed in the United States in 1995. The licensed vaccine is a preparation of the Oka strain of varicella virus obtained from the vesicle fluid of a healthy child with varicella that has been attenuated by serial propagation in human embryo lung fibroblasts, guinea pig embryonic cells, and human diploid cell cultures. Varicella vaccine is labile and must be stored at 4°C for less than 72 h or frozen at 15°C or colder until reconstituted.

Varicella vaccine is highly immunogenic in susceptible children. Seroconversion has occurred in more than 96% of children 12 months to 12 years of age after one dose of vaccine (372). Preexisting antibody, if present at 12 months of age, does not appear to interfere with antibody response. As with other viral vaccines, the antibody response after immunization is lower than that from natural disease. Adolescents and adults have age-related decreases in the ability to develop a primary response to varicella virus (186). Seroconversion rates of 78 to 82% after one dose and 99% after two doses have been reported in those older than 12 years (186, 372).

In ongoing studies in the United States and Japan, serum antibodies to varicella have been detected for as long as 10 to 20 years after immunization in more than 95% of immunized children (34, 222). Antibody concentrations have persisted for at least 1 year in 97% of adults and adolescents who were given two doses of vaccine 4 to 8 weeks apart (186). Cell-mediated immunity to varicella-zoster virus (VZV) has been detected in 87% of children and 94% of adults at 5 years postvaccination (377).

Varicella vaccine is highly effective in preventing varicella in children and in reducing the severity of infection if they do become infected. In prelicensure controlled clinical trials, varicella vaccine was 70 to 90% effective in preventing varicella and more than 95% effective in preventing severe disease (242, 368). Several postlicensure studies have shown similar results, with vaccine effectiveness ranging from 83 to 100% in preventing varicella and 87 to 100% in preventing severe disease (129, 212, 362). In follow-up studies, about 1 to 4% of vaccinated children per year have developed chickenpox following exposure to wild-type varicella virus, a rate that does not seem to increase with length of time after immunization (128). These vaccine failure-related cases are mild, with fewer skin lesions, lower rates of fever, and more rapid recovery (47, 367).

In adults and adolescents who have seroconverted, varicella vaccine provides protective efficacy rates of approximately 70% after household exposure. The remaining 30% develop attenuated disease with fewer skin lesions and little or no systemic toxicity, as in children (186).

The use of varicella vaccine has an impact on the epidemiology of disease. Active surveillance for varicella has been conducted at sites in Pennsylvania, Texas, and California since 1995 as part of a CDC-sponsored study (J. Seward, J., C. Peterson, L. Mascola, et al., Abstr. Pediatr. Acad. Soc. Am. Acad. Pediatr. Joint Meet., abstr. 1629, 2000). During the period from 1995 to 1999, vaccine coverage in 1- to 2-year old children rose to 70% while overall cases of varicella declined by 70 to 90%. The greatest decline was in children 1 to 4 years of age, but cases also declined in all other age groups, including infants younger than 1 year and adults, suggesting herd immunity. Decreasing rates of varicella have also been associated with increasing use of varicella vaccine in a day care center population (127).

Current estimates of vaccine efficacy and antibody persistence in vaccinees are based on observations when natural varicella infection has been highly prevalent. The extent to which boosting from exposure to natural varicella has impacted on the efficacy of vaccine or duration of immunity is not known. In prelicensure clinical studies, mean serum anti-VZV levels among vaccinees continued to increase with time after vaccination. This has been attributed to immunologic boosting caused by exposure to wild-type VZV in the community. A recent study analyzed serum antibody levels and infection rates over 4 years of follow-up in 4,631 children immunized with varicella vaccine (240). Anti-VZV titers decreased over time in high-responder subjects but rose in vaccinees with low titers. Among subjects with low anti-VZV titers, the frequency of clinical infection and immunological boosting substantially exceeded the 13%-per-year rate of exposure to wild-type VZV. These findings suggest that vaccine strain VZV persisted in vivo and reactivated as serum antibody titers decreased after vaccination.

Varicella vaccine produces relatively few adverse reactions (331, 374). Local reactions, rashes, and low-grade fevers occur in as many as 10% of vaccine recipients (32), but rates of rash and fever have been similar in placebo groups in several studies (169, 368). In postlicensure studies, the most frequently reported adverse event is a mild vesicular rash that occurs in approximately 5% of vaccinees (97, 185). Vesicular rashes that occurred within 2 weeks of vaccination were more likely to be due to wild-type VZV, while rashes that occurred more than 2 weeks postvaccination were more likely to be due to the Oka vaccine strain (331). There has been one report of rash and pneumonia as the result of vaccination of a 15-month-old child infected with human immunodeficiency virus (HIV). The child's HIV status was not known at the time of vaccination (374).

A major concern has been whether vaccination would increase the risk of zoster. Based on reports to VAERS, the rate of herpes zoster after varicella vaccination is 2.6/100,000 vaccine doses distributed (97). The incidence of herpes zoster after natural varicella infection among healthy persons younger than 20 years is 68/100,000 person-years (194) and, for all ages, 215/100,000 person year (160). However, these rates should be compared cautiously because the latter rates are based on populations monitored for longer periods than were the vaccinees. Cases of herpes zoster have been confirmed by PCR to be caused by both vaccine virus and wild-type virus, suggesting that some herpes zoster cases in vaccinees might result from antecedent natural varicella infection (97, 199).

Transmission of the vaccine virus is rare and occurs most often from immunocompromised vaccinees. Of the 15 million doses of varicella vaccine distributed, on only three occasions has transmission from immunocompetent persons been documented by PCR analysis (244, 321). All three cases resulted in mild disease without complications. In one case, a child aged 12 months transmitted the vaccine virus to his pregnant mother (254, 321). The mother elected to terminate the pregnancy, and fetal tissue tested by PCR was negative for varicella vaccine virus. The two other documented cases involved transmission from healthy children aged 1 year to a healthy sibling aged 41/2 months and a healthy father, respectively (97). Transmission has also occurred from patients with zoster due to vaccine strain virus (56). Transmission has not been documented in the absence of a vesicular rash postvaccination. No evidence indicates reversion to virulence of the vaccine strain during transmission; siblings of leukemic vaccine recipients who acquired vaccine virus had mild rashes in 75% of cases and symptomless seroconversion in 25% (32).

Varicella vaccine is licensed for use in individuals 12 months of age or older who have not had varicella. One dose of varicella vaccine is recommended for immunization of susceptible healthy children from 12 to 18 months of age (23, 96). The vaccine may be given concurrently with measles-mumps-rubella vaccine (MMR) but at separate sites. In addition, one dose of vaccine is recommended for immunization of all children from age 19 months to the 13th birthday who lack a reliable history of varicella infection and who have not previously been vaccinated. Susceptible healthy adolescents who have reached their 13th birthday and adults should be immunized with two doses of varicella vaccine 4 to 8 weeks apart. If more than 8 weeks elapses following the first dose, the second dose can be administered without restarting the schedule. If the adolescent or young adult does not have a reliable history of varicella, serologic testing for immunity before vaccination is likely to be cost-effective since 71 to 93% of such individuals are actually immune (251).

In February 1999, the ACIP expanded its recommendations for varicella vaccine to promote wider use of the vaccine for susceptible children and adults (97). The updated recommendations include establishing child care and school entry requirements, use of the vaccine following exposure and for outbreak control, use of the vaccine for some children infected with HIV, and vaccination of adults and adolescents at high risk for exposure.

The ACIP recommends that all states require children entering child care facilities and elementary schools to have received varicella vaccine or to have other evidence of immunity to varicella. Other evidence of immunity should consist of a physician's diagnosis of varicella, a reliable history of the disease, or serologic evidence of immunity. To prevent susceptible older children from entering adulthood without immunity to varicella, states should also consider implementing a policy that requires evidence of varicella vaccination or other evidence of immunity for children entering middle school or junior high school.

Data from both the United States and Japan indicate that varicella vaccine is effective in preventing illness or modifying varicella severity if used within 3 days, and possibly up to 5 days, of exposure (32, 35, 320, 366). The ACIP and the AAP now recommend the vaccine for use in susceptible persons following exposure to varicella (10, 97). If the exposure results in infection, no evidence indicates that administration of varicella vaccine during the presymptomatic or prodromal stage of illness increases the risk for vaccine-associated adverse events.

Varicella outbreaks in child care facilities, schools, and other institutions can last 3 to 6 months. Varicella vaccine has been used successfully by state and local health departments and by the military for outbreak prevention and control. Therefore, the ACIP recommends that state and local health departments consider using the vaccine for outbreak control either by either advising exposed susceptible persons to contact their health care providers for vaccination or offering vaccination through the health department (97).

The ACIP has strengthened its recommendations for susceptible persons aged 13 years or older at high risk for exposure or transmission, including designating adolescents and adults living in households with children as a new high-risk group (97). Varicella vaccine is recommended for susceptible persons in the following high-risk groups: (i) persons who live or work in environments where transmission of VZV is likely, (ii) persons who live and work in environments where transmission can occur, (iii) nonpregnant women of childbearing age, (iv) adolescents and adults living in households with children, and (v) international travelers. Vaccination is also routinely recommended for all susceptible health care workers (77).

Varicella vaccine is not licensed for use in persons who have blood dyscrasias, leukemia, lymphomas of any type, or other malignant neoplasms affecting the bone marrow or lymphatic systems. The manufacturer makes free vaccine available to any physician through a research protocol for use in patients who have acute lymphoblastic leukemia and who meet certain eligibility criteria (96). The ACIP previously recommended that varicella vaccine not be administered to persons with primary or acquired immunodeficiency, including immunosuppression associated with AIDS or other clinical manifestations of HIV infections, cellular immunodeficiencies, hypogammaglobulinemia, and dysgammaglobulinemia (96). The ACIP maintains its recommendation that varicella vaccine not be administered to persons who have cellular immunodeficiencies, but persons with impaired humoral immunity may now be vaccinated (97). In addition, some HIV-infected children may now be considered for vaccination. Limited data from a clinical trial in which two doses of varicella vaccine were administered to 41 asymptomatic or mildly symptomatic HIV-infected children (CDC class N1 or A1, age-specific CD4⁺ T-lymphocyte percentage of 25%) (66) indicated that the vaccine was immunogenic and effective (97; Pediatric AIDS Clinical Trial Group, unpublished data). Because children infected with HIV are at increased risk for morbidity from varicella and herpes zoster compared with healthy children, the ACIP recommends that, after weighing potential risks and benefits, varicella vaccine should be considered for asymptomatic or mildly symptomatic HIV-infected children in CDC class N1 or A1 with age-specific CD4⁺ T-lymphocyte percentages of 25% (97). Eligible children should receive two doses of varicella vaccine with a 3-month interval between doses. Because persons with impaired cellular immunity are potentially at greater risk for complications after vaccination with a live vaccine, these vaccinees should be encouraged to return for evaluation if they experience a postvaccination varicella-like rash. The use of varicella vaccine in other HIV-infected children is being investigated.

Recommendations regarding the use of varicella vaccine in persons with other conditions associated with altered immunity or in persons receiving steroid therapy have not changed. Varicella vaccine is contraindicated in the following situations: (i) pregnancy, (ii) severe febrile illness, and (iii) known history of anaphylactic reaction to vaccine components (23, 96). Administration of varicella vaccine should be avoided if the individual is receiving immunosuppressive doses of systemic corticosteroids. The effects of corticosteroids vary, but many clinicians consider a prednisone dose equivalent to either 2 mg/kg of body weight or 20 mg per day to be sufficiently immunosuppressive to raise concerns about the safety of vaccination with live-virus vaccines.

After cessation of immunosuppressive therapy, varicella vaccine is generally withheld for at least 1 month. Because the intensity and type of immunosuppressive therapy, radiation therapy, underlying disease, and other factors determine when immunologic responsiveness will be restored, however, a definitive recommendation for an interval after cessation of immunosuppressive therapy when varicella vaccine can be safely and effectively administered is often not possible.

Transmission of the live attenuated varicella vaccine virus used for immunization has been documented rarely. Therefore, contacts of immunocompromised patients should be vaccinated to prevent the spread of natural varicella to such patients. Vaccinees who develop a rash in the month after immunization should avoid direct contact with immunocompromised, susceptible individuals for the duration of the rash.

Receipt of antibody-containing blood products (whole blood, plasma, or parenteral immunoglobulin) may interfere with seroconversion to varicella vaccine. Varicella vaccine should not be given within at least 5 months after administration of immune globulin or blood transfusion.

Reye's syndrome has occurred in children infected with varicella who receive salicylates. Whether varicella vaccine might induce Reye's syndrome is not known, but the vaccine manufacturer recommends that salicylates should not be given within at least 6 weeks after administration of varicella vaccine.

DISEASES FOR WHICH IMPROVED VACCINES BECAME AVAILABLE

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Haemophilus influenzae Type b

Prior to the introduction of routine infant and childhood vaccination against Haemophilus influenzae type b (Hib), this pathogen was a major cause of invasive bacterial infections in young children in the United States. It was the most common cause of bacterial meningitis and epiglottitis and a significant cause of septic arthritis, occult febrile bacteremia, and pneumonia in children younger than 5 years, causing an estimated 12,000 cases of meningitis and 8,000 additional cases of invasive Hib disease annually (131). The cumulative risk of Hib disease was approximately 1 in every 200 American children in the first 5 years of life, with the peak incidence of Hib meningitis occurring between 6 and 12 months of age.

Because the majority of cases of Hib disease occurred in infancy, vaccines that induced protection by 6 months of age were necessary for effective control of Hib disease. Realization of this goal was made possible by the development of conjugate vaccines in which the capsular polysaccharide of Hib, its major virulence factor and the antigen against which protective antibodies are directed, is chemically linked to a protein carrier. Although the purified capsular polysaccharide, polyribosylribitol phosphate (PRP), is a poor immunogen in children younger than 18 months, PRP conjugated to a protein carrier has the antigenic properties of the protein carrier. As a result, PRP conjugate vaccine induces protective antibody in infants and young children and significantly greater concentrations of circulating anti-PRP at all ages than does the unconjugated polysaccharide (315, 370). This age-dependent immunogenic characteristic of purified PRP (and other polysaccharide antigens) is that of a T-cell-independent antigen to which humoral responses are mediated by B-cell lymphocytes alone without T-helper lymphocytes. In contrast, the polysaccharide-protein conjugate vaccines are T-cell-dependent antigens in which T-helper lymphocyte activation as well as B-cell mediation of the humoral antibody response occurs. T-cell-dependent antigens also elicit booster responses that are important to the effectiveness of polysaccharide vaccines.

Four conjugate vaccines have been licensed in the United States since 1987 (Table 3). Each conjugate vaccine is composed of PRP antigen conjugated to a protein carrier. The vaccines differ in the protein carrier, size of the saccharide component, and chemical linkage. The PRP-D conjugate vaccine is licensed only for infants 15 months of age or older. Three Hib vaccines are licensed for infant vaccination: HbOC, PRP-T, and PRP-OMP.

Conjugate vaccines are immunogenic in infants and young children (152, 189, 370). Children 15 months of age or older respond well to a single dose of any of the four conjugate vaccines. In infants, the immunogenicity of the conjugate vaccines differs according to the product, age of vaccination, and number of doses (152, 189, 370). While PRP-OMP induces significant increases in antibody concentration after a single injection at 2 months of age, the other three vaccines do not. For all three vaccines licensed for use in infants, two or three doses in the first 6 months of life result in high rates of seroconversion (50, 152, 189).

Placebo-controlled field trials of both HbOC and PRP-OMP in infants in the United States demonstrated nearly 100% protection and provided the basis for the initial approval of these vaccines for use in this country. In a study in northern California of HbOC, vaccine efficacy was 100% for infants receiving the three-dose schedule at 2, 4, and 6 months of age (50). In a Navajo population of infants at high risk of Hib disease who were vaccinated at 2 and 4 months of age with either PRP-OMP or placebo, vaccine efficacy was 100% at 1 year of age and 93% in total (324). Randomized, placebo-controlled trials of PRP-T were terminated before completion, when the FDA approved HbOC and PRP-OMP for use in infants (183). Licensure of PRP-T was based on the comparable immunogenicity in a three-dose schedule to that of the other two products. In addition, an efficacy trial in Great Britain and the lack of cases in the terminated trials indicate comparable efficacy of PRP-T to that of HbOC and PRP-OMP (52, 359).

Hib conjugate vaccines are well tolerated. Local reactions occur in approximately 25% of recipients but typically are mild and last less than 24 h (152, 370). Systemic reactions such as fever and irritability are infrequent. While an increased risk of invasive Hib disease in the early postvaccination period exists with unconjugated PRP vaccine, the risk of disease immediately after conjugate vaccination is not increased (209). Other serious adverse events, such as anaphylaxis, have not been reported with Hib conjugate vaccines.

Since 1991, routine vaccination against Hib disease has been recommended for all children beginning at approximately 2 months of age (13, 59, 101). HbOC, PRP-T, and PRP-OMP are now considered interchangeable for primary as well as booster vaccination. Excellent immune responses have been achieved when vaccines from different manufacturers have been interchanged in the primary series (29, 48, 190). If PRP-OMP is administered in a series with one of the other two products licensed for infants, the recommended number of doses to complete the series is determined by the other product (and not by PRP-OMP). For example, if PRP-OMP is administered for the first dose at age 2 months and another vaccine is administered at age 4 months, a third dose of any of the three licensed Hib vaccines is recommended at age 6 months to complete the primary series. A final dose of any product, irrespective of the prior vaccines received, is acceptable at 12 to 15 months of age for completion of the Hib immunization schedule.

For children in whom Hib immunization has not been initiated by 7 months of age, recommended schedules differ according to the child's age and choice of conjugate vaccine (see references 13 and 59 for further information). Previously unimmunized children aged 15 months or older should be immunized with a single dose of any licensed conjugate Haemophilus vaccine. For previously unimmunized children 5 years or older, immunization is indicated only if they have an underlying condition predisposing to Hib disease, such as asplenia or HIV infection.

Introduction of Hib conjugate vaccines in the United States, first in children 18 months and older and later as a routine infant immunization, has dramatically decreased the incidence of disease. By 1995, Hib disease levels had declined by more than 95% below preimmunization levels (98, 369). The remarkably rapid reduction in disease incidence was partly because of the ability of the vaccine to reduce nasopharyngeal carriage of the organism, leading to reduced rates of exposure and infection even in those not immunized (39).

Pertussis (whooping cough) continues to cause significant morbidity and mortality among young children worldwide (275). The WHO has estimated that in the absence of vaccination, approximately 10⁶ deaths would have occurred from the disease and its complications. In the United States, the number of cases has been reduced by approximately 95% during the vaccine era. Nevertheless, approximately 4,000 reported cases of pertussis still occur each year (86) and large outbreaks of 100 or more cases also have occurred recently (122, 177).

Effective prevention programs necessitate immunization of young infants, beginning at 2 months of age, because the morbidity and mortality of pertussis is greatest in infants, especially those younger than 6 months (86, 120). Approximately 35% of reported cases in the United States occur in infants younger than 6 months. In this age group, the case-fatality rate from 1992 to 1994 was 0.6%; 71% of the infants were hospitalized; and complications such as pneumonia, seizures, and encephalopathy were frequent (86). High rates of immunization in children beyond infancy may further reduce the risk of infection in infants by decreasing the incidence of infection in older family members and the transmission of Bordetella pertussis within the household.

Whole-cell pertussis vaccine, which has been in use for many years, is a suspension of inactivated B. pertussis, and is combined with diphtheria and tetanus toxoids (DTP). This vaccine has a high incidence of local and systemic reactions. To reduce the incidence of reactions, acellular vaccines composed of one or more purified components of B. pertussis have been developed and combined with diphtheria and tetanus toxoids (DTaP). Multiple acellular vaccines have been formulated from different components of B. pertussis and have been tested in children. All vaccines contain a detoxified pertussis toxin, pertussis toxoid (151). In addition, most vaccines have one or more of the following B. pertussis antigens: filamentous hemagglutinin, pertactin, and fimbrial proteins. In the early 1990s, two acellular vaccines were approved for use in children 15 months of age and older in the United States. Approval for infant immunization followed in 1996. Four acellular vaccines are approved for the primary vaccination series during infancy (Table 4). Two of these vaccines, ACEL-IMUNE and Certiva, were withdrawn from the market in 2001. Of the available acellular pertussis vaccines, Tripedia is currently licensed for the five-dose DTaP vaccination series while Infanrix is licensed for the first four doses of the vaccination series. The ACIP recommends that whenever feasible, the same brand of DTaP vaccine should be used for all doses of the vaccination series. If the vaccine provider does not know or does not have available the type of DTaP previously administered, any of the available licensed DTaP vaccines may be used to complete the vaccination series (114).

For both whole-cell and acellular vaccines, serological correlates of immunogenicity have not been established for assessing efficacy. As a result, field and other epidemiological studies are necessary to demonstrate efficacy. Studies of household contacts exposed to pertussis in the United States indicate that the efficacy of whole-cell vaccine is 80% or greater (86, 120, 286). The efficacy of eight acellular pertussis vaccines in infants has been evaluated (302). Rates of prevention of pertussis with these vaccines have ranged from 58 to 93%. Comparison of efficacy between the different products, however, often is not possible because of differences in study design, vaccine schedule (specifically, number of doses and age of administration), case definitions for pertussis, and other confounding variables. In general, these acellular vaccines appear to be similar in efficacy to most whole-cell vaccines.

Local and febrile reactions to whole-cell vaccines are common, occurring in more than half of DTP recipients (132). These manifestations usually develop within the first 24 h and are of brief duration. The incidence of these febrile reactions following administration of acellular vaccine is significantly lower (151, 302). Comparison of the rates with different acellular vaccines have demonstrated similar safety profiles for each of these vaccines (123, 329). Rates of local reactions increase with each subsequent dose of DTaP vaccine (298, 356). Booster doses of acellular pertussis vaccine may be associated with extensive local swelling, especially with vaccines containing high diphtheria vaccine content (309). Severe reactions to acellular vaccines are rare (123, 151, 302, 329).

Vaccination against pertussis is routinely recommended for children at 2, 4, and 6 months of age, followed by a fourth dose at 12 to 18 months of age and a fifth dose at 4 to 6 years of age (8, 11, 87, 88). As of January 2000, the AAP and ACIP recommend exclusive use of acellular pertussis vaccines for all doses of the pertussis vaccine series (85, 114). DTP is not an acceptable alternative because of its higher rates of local reactions, fever, and other common systemic reactions.

Contraindications and precautions for pertussis vaccine are based on adverse reactions associated with whole-cell vaccine. While reactions following DTaP are much less common than those associated with DTP, the contraindications and precautions for DTaP are currently the same (8, 11, 87, 88).

Streptococcus pneumoniae is the most common cause of invasive bacterial infection in children. In addition, the organism is responsible for 30 to 50% of cases of acute otitis media (AOM). Most cases of invasive disease occur in children younger than 2 years and adults 65 years of age and older (95).

To date, at least 90 serotypes of S. pneumoniae have been identified. The 23-valent polysaccharide pneumococcal vaccine, licensed in 1983, contains purified capsular polysaccharide antigen of 23 serotypes. These serotypes are responsible for 85 to 90% of adult infections and nearly 100% of cases of invasive disease and 85% of cases of otitis media in children. Efficacy rates for prevention of bacteremia and meningitis caused by vaccine serotypes for the polysaccharide vaccine are 61 to 75% in immunocompetent adults (58, 330) and 57% in children younger than 5 years (58).

The polysaccharide vaccine has been effective in reducing severe disease in the adult population (58) but has had little impact in young children because the vaccine is not immunogenic in children younger than 2 years of age (9). In addition, the polysaccharide vaccine has not been effective in preventing otitis media caused by S. pneumoniae (9).

Recommendations by the ACIP and the AAP do not include routine use of the currently licensed polysaccharide vaccine except for those older than 2 years with a medical condition which puts them at high risk of severe pneumococcal infection, such as sickle cell disease, functional or anatomic asplenia, nephrotic syndrome or chronic renal failure, immunodeficiency, cerebrospinal fluid leak, or HIV infection, and adults older than 65 years (9, 95).

Several factors have made the development of new preventative strategies for pneumococcal disease a high priority (137). Resistance of S. pneumoniae to multiple antibiotics has increased rapidly in the United States and even more rapidly in other parts of the world (224). Children younger than 2 years have the highest rate of invasive pneumococcal infection but do not develop an effective antibody response to polysaccharide vaccine. In addition, children up to 5 years of age may have poor responses to serotypes 6B, 14, 19F, and 23F, common causes of pediatric infections and the most prevalent penicillin-resistant serotypes.

These factors have prompted the development of conjugated polysaccharide-protein vaccines. These new vaccines are similar in design to the licensed Hib conjugate vaccines. Pneumococcal conjugate vaccines under development differ in the carrier protein, the molecular size of the polysaccharide, and the method of conjugating the polysaccharide to the protein (170). To date, the candidate carrier proteins have been the same as those used in Hib conjugate vaccines. Because of the large number of serotypes of S. pneumoniae which cause disease, development of these vaccines has been more difficult than development of similar vaccines for Hib. Each pneumococcal antigen must be coupled to a protein carrier, and the vaccine must be prepared to ensure that enough antigen is present to induce an immune response but not enough to elicit an adverse reaction.

Conjugate vaccines that contain polysaccharides of 5, 7, 9, or 11 pneumococcal serotypes conjugated to either tetanus toxoid, diphtheria toxoid, meningococcal outer membrane complex, or a mutant diphtheria toxin (CRM₁₉₇) have been developed (170). Pneumococcal conjugate vaccines appear to be safe. The most commonly reported reactions have been local reactions at the injection site, but these occur at a lower frequency than do local reactions with other childhood vaccines such as DTP (310).

Currently, five pneumococcal vaccine candidates with different carrier proteins are under development (170). All appear to be comparable in their ability to induce primary immunity and immunologic memory. The immunogenicity of the vaccine appears to determined by the pneumococcal polysaccharide serotype rather than the carrier protein. Some serotypes (14, 18C, and 19F) are excellent immunogens, eliciting antibody protection after a single dose, while others (6B and 23F) require three doses of vaccine (170). All conjugate vaccines appear to elicit immunologic memory (170). Antibody concentrations achieved after the initial series of three doses are usually sustained only for a few months and then decline to near preimmunization levels. A dose of pneumococcal vaccine, either polysaccharide or conjugate, given in the second year of life elicits an amnestic-type response.

Heptavalent vaccine containing serotypes 4, 6B, 9V, 14, 18C, 19F, and 23F conjugated to CRM₁₉₇ was studied in a large prospective placebo-controlled efficacy trial in Northern California that involved 38,000 children. The vaccine was 89% effective in prevention of invasive disease caused by any pneumococcal serotype and 97% effective against disease due to the seven vaccine serotypes (49). For noninvasive disease, there was a 7% decrease in cases of otitis media and a decrease of 23% in doctor visits for recurrent otitis (at least six visits per year) in vaccinated children. The study also demonstrated a 11% decrease in clinical cases of pneumonia in vaccinees (H. Shinefield, 17th Eur. Soc. Pediatr. Infect. Dis. Meet. abstr. PS 14, 1999).

The ability of the heptavalent vaccine to protect children against AOM was also evaluated in an efficacy trial conducted in Finland (171). Efficacy was estimated to be 34% for prevention of AOM caused by pneumococci of any serotype, while efficacy against AOM irrespective of etiology was 6%. Efficacy against AOM caused by vaccine-related serotypes was 57%, however, an increase of 33% in the rate of AOM episodes caused by nonvaccine serotypes occurred in the group receiving the heptavalent vaccine compared with controls. However, in spite of the increase in disease caused by nonvaccine serotypes, the net effect on pneumococcal AOM was a reduction of 34%.

The heptavalent vaccine (Prevnar [Wyeth-Lederle Vaccines]) was licensed in the United States in February 2000. Recommendations by the AAP and the ACIP for use of the licensed heptavalent pneumococcal conjugate vaccine (PCV7) have been issued (22, 90). The AAP and ACIP recommend universal use of PCV7 in children 23 months and younger. For children in whom pneumococcal immunization is initiated before 7 months of age, four doses of PCV7 are recommended at 2, 4, 6, and 12 to 15 months of age. For children beginning PCV7 immunization between 7 and 23 months of age, the recommended schedule differs according to the child's age (see references 22 and 90 for further information). In addition, two doses of PCV7 are recommended for children 24 to 59 months of age who are at high risk of invasive pneumococcal infection and have not previously been immunized with PCV7. These children should also receive the 23-valent polysaccharide vaccine (PPV23) to expand serotype coverage. Routine immunization of low- and moderate-risk children 24 months of age or older is not recommended by the AAP at this time (22). The ACIP recommends that PCV7 be considered for all children aged 24 to 59 months, with priority given to those aged 24 to 35 months, of Alaskan Native, American Indian, or African American descent, and those who attend group day care centers (90). Children aged 24 to 59 months at high risk who have not previously received PCV7 but who have already received PPV23 should be vaccinated with two doses of PCV7 given 2 months apart (90). Current data do not support a recommendation to replace the 23-valent polysaccharide vaccine with PCV7 vaccine for older children and adults (90).

Routine infant immunization using this newly developed conjugate vaccine could lead to significant reductions in S. pneumoniae disease. If use of the conjugate vaccine eliminates nasopharyngeal carriage of S. pneumoniae, as suggested by preliminary data suggests (146, 147), person-to-person transmission will be interrupted and the incidence of disease should decline markedly.

One potential problem with such vaccines is the need to immunize against many different serotypes of pneumococci. Because of local reactions to the protein component, conjugate vaccines which contain more than 12 serotypes may be difficult to produce. As a result, different formulations of conjugate pneumococcal vaccine may be developed that would contain different serotypes targeted for a specific group of patients. Vaccine containing types 4, 6B, 9V, 14, 18C, 19F, and 23F would be necessary for prevention of otitis media in the United States, while types 1, 2, and 5 would need to be added to prevent pneumonia in developing countries. In addition, the use of conjugate vaccines against limited serotypes may lead to the emergence of pneumococcal serotypes which are currently less common and require adjustment of the vaccine composition.

While the likelihood of human exposure to a rabid domestic animal in the United States is small, international travelers to areas where canine rabies is still endemic have an increased risk of exposure to rabies. In most of Asia, Africa, and Latin America, dogs are the most common source of rabies among humans. Of the 36 human rabies deaths reported to the CDC from 1980 through 1997, 12 appear to have been related to rabid animals outside the United States (75, 283).

Four formulations of three inactivated rabies vaccines are currently licensed for preexposure and postexposure prophylaxis in the United States. Human diploid cell vaccine (HDCV), derived from the Pitman-Moore strain, has been licensed in the United States since 1980. It is supplied in two forms: for intramuscular (I.M.) administration or for intradermal administration (63). Rabies vaccine, adsorbed (RVA), derived from the Kissling strain of rabies virus cultured in fetal rhesus lung diploid cells, was licensed in the United States in 1988. It is formulated for I.M. administration only. A third rabies vaccine, purified chick embryo cell (PCEC) vaccine, became available in the United States in 1997 (161). It is prepared from the Flury LEP rabies virus strain grown in primary cultures of chicken fibroblasts and is formulated for I.M. administration only. Duck embryo rabies vaccine has not been available in the United States since 1981. Allergic reactions occurred frequently with this vaccine.

When used as indicated, all three types of rabies vaccines are considered equally safe and effective for both preexposure and postexposure prophylaxis (see the ACIP recommendations [74]). However, only the HDCV intradermal vaccine has been evaluated and approved by the FDA for the intradermal dose and route for preexposure vaccination (63). Therefore, the RVA and PCEC vaccines should not be used intradermally. Usually, an immunization series is initiated and completed with one vaccine product. No clinical studies have been conducted that document a change in efficacy or the frequency of adverse reactions when the series is completed with a second vaccine product.

For adults, the i.m. rabies vaccination should always be administered in the deltoid area. For children, the anterolateral aspect of the thigh is also acceptable. The gluteal area should never be used for HDCV, RVA, or PCEC injections because administration of HDCV in this area results in lower neutralizing antibody titers (D. B. Fishbein, L. A. Sawyer, F. L. Reid-Sanden, and E. H. Weir. Letter, N. Engl. J. Med. 318:124-125, 1988).

Reactions after vaccination with HDCV, RVA, and PCEC vaccines are less serious and less common than with previously available vaccines (74). Local reactions at the injection site occur in 30 to 74% of injections, and systemic reactions, such as headache, nausea, abdominal pain, muscle aches, and dizziness, occur in 5 to 40% of vaccine recipients. Approximately 6% of persons who received booster doses of HDCV developed an immune complex-like reaction 2 to 21 days after administration of the booster dose (65). This reaction occurred less frequently among persons receiving primary vaccination. The reactions have been associated with the presence of betapropiolactone-altered human albumin in the HDCV and the development of immunoglobulin E (IgE) antibodies to this allergen (180).

The essential components of rabies postexposure prophylaxis are wound treatment and, for previously unvaccinated persons, the administration of both rabies immune globulin (RIG) and vaccine (74). Studies conducted in the United States by CDC have documented that a regimen of one dose of RIG and five doses of rabies vaccine over a 28-day period was safe and induced an excellent antibody response in all recipients (30). RIG provides a rapid, passive immunity that persists for only a short time (half-life, approximately 21 days). The recommended dose of human RIG is 20 IU/kg of body weight. If anatomically feasible, the full dose of RIG should be thoroughly infiltrated in the area around and into the wounds. Any remaining volume should be injected intramuscularly at a site distant from vaccine administration. RIG is unnecessary and should not be administered to previously vaccinated persons because an amnestic response will follow the administration of a booster regardless of the antibody titer (74).

Once initiated, rabies prophylaxis should not be interrupted or discontinued because of local or mild systemic adverse reactions to rabies vaccine. Usually, such reactions can be successfully managed with anti-inflammatory and antipyretic agents, such as ibuprofen or acetaminophen. When a person with a history of serious hypersensitivity to rabies vaccine must be revaccinated, antihistamines can be administered. Epinephrine should be readily available to counteract anaphylactic reactions, and the person should be observed carefully immediately after vaccination.

Parenteral inactivated vaccines have been used for many years. In field trials of inactivated typhoid vaccine, vaccine efficacy has ranged from 51 to 76% (109). Protection has not been correlated with specific Salmonella enterica serovar Typhi antibodies. In addition, protection in experimental challenge studies in volunteers has varied with the challenge inoculum and can be overcome by a high inoculum of serovar Typhi (248). Following parental vaccination, febrile reactions, headache, and severe local pain and swelling are common; 13 to 24% of vaccinees have subsequently missed school or work (248). Booster doses are recommended for recipients of parenteral vaccines.

Two new typhoid vaccines that provide significant protection without causing adverse reactions have been licensed in many countries (248). One of these is Ty21a, a live attenuated oral vaccine, which was licensed in the United States in 1991. The other is a parenteral vaccine containing the purified Vi polysaccharide capsular antigen of serovar Typhi which was licensed in 1994.

The oral vaccine consists of a stable mutant of a pathogenic serovar Typhi strain, Ty21a, that lacks the enzyme UDP-galactose-4-epimerase and the Vi capsular polysaccharide. The efficacy of the oral Ty21a vaccine in clinical trials has ranged from 42 to 96% following the initial series of three doses, with the lower efficacies seen in trials from areas with highly endemic disease (248, 334). The mechanism of protection is not known. Reactions from the oral Ty21a vaccine have been mild and rare. In safety trials, adverse reactions occurred with equal frequency among groups receiving vaccine or placebo (248). Since data on safety and efficacy in young children are not available, the manufacturer currently recommends that Ty21a vaccine not be given to children younger than 6 years.

Recommendations for booster doses for those whose primary immunization was given with oral vaccines have not yet been determined. Primary immunization involves multiple doses and varies in schedule and dose according to the vaccine preparation. Since the oral vaccine is a live attenuated strain, it should not be given to immunocompromised persons, including those known to be infected with HIV (248). This vaccine is promising for the control of endemic typhoid fever because protection lasts for at least 5 years and mass immunization may result in herd immunity.

Purified Vi, a recognized virulence factor of serovar Typhi, has been used as a parenteral polysaccharide vaccine. Efficacy of the Vi vaccine in clinical trials is 72% at 17 months and 64% at 21 months following a single dose (248). Since additional doses of purified Vi vaccine fail to boost antibody titers, Robbins and Schnesson have conjugated the Vi polysaccharide to protein carriers such as Escherichia coli labile toxin to confer T-cell-dependent properties upon the antigen (315). A clinical trial of a conjugate of the Vi polysaccharide bound to nontoxic recombinant Pseudomonas aeruginosa exotoxin A in Vietnamese children aged 2 to 5 years found the vaccine to be safe and immunogenic (252). The efficacy of this conjugate vaccine was 91%.

In developing recommendations for immunization, multiple factors are considered, including vaccine characteristics, scientific knowledge about the principles of immunization, assessment of the benefits of the vaccine, risk of the disease and its complications, vaccine costs, and risks of adverse reactions. Changes in relative benefits and risks necessitate continued review of recommendations. In the United States, recommendations for immunization of infants and children are made by two different committees, the ACIP and the AAP Committee on Infectious Diseases. These committees work closely together, and in most circumstances their recommendations are similar. Since 1995, these two committees and the American Academy of Family Practice (AAFP) have issued a single vaccine schedule at least once a year (103).

Several major changes in immunization schedules and strategy have occurred in the 1990