Impact of Conjugate Pneumococcal Vaccines

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Abstract
Pneumococcal capsular polysaccharide has been available for decades but does not induce protective immune responses in infants and toddlers who are at highest risk for disease. In February 2000, a protein-polysaccharide conjugate vaccine (Prevnar; Wyeth Pharmaceuticals, Philadelphia, PA), including serotypes 4, 6B, 9V, 14, 18C, 19F and 23F, was licensed for use in infants and young children and recommended as part of the routine U.S. childhood immunization schedule.1 The pivotal efficacy trial for licensure of this vaccine, conducted in Northern California, demonstrated efficacy of 97.4% against invasive pneumococcal disease caused by serotypes contained in the vaccine.2 Other efficacy trials conducted with this vaccine and other pneumococcal conjugate vaccine (PCV) products have shown significant efficacy against a variety of microbiologic and clinical outcomes including invasive pneumococcal disease, nasopharyngeal colonization, otitis media, pneumonia and antimicrobial resistance. Data are now available on the impact of vaccine on colonization and disease with routine use. Studies evaluating PCV effects on nasopharyngeal carriage demonstrate that vaccination does not impact the overall risk of pneumococcal NP colonization but reduces the acquisition of vaccine-type strains and increases the risk of non-vaccine serotype strain acquisition.3 Among toddlers in Israel, vaccination reduced carriage of antibiotic-resistant strains in vaccinated children and in their unvaccinated siblings through reduced transmission.4 A study of carriage in the United States after routine infant immunization at 2, 4, 6 and 12 months found that carriage of vaccine-type pneumococci decreased and non-vaccine-type carriage increased after the fourth dose.5 Four controlled trials have assessed the effect of vaccine on invasive disease (Table 1). The populations studied include a general U.S. population (Northern California2,6), a high risk U.S. population (American Indians7), an urban African population with high HIV seroprevalence (Soweto, South Africa8) and a rural African population with low HIV seroprevalence but endemic for malaria (The Gambia9). All trials have shown efficacy of the vaccine against serotype-specific invasive disease of >75%. All 4 of these trials have also assessed the effect of vaccine on clinically and radiologically diagnosed pneumonia (Table 1; American Indian trial pneumonia results presented only in abstract form). In the Northern California trial, vaccination reduced episodes of pneumonia confirmed by radiograph by 20.5% (Table 1).6 Vaccine effect on x-ray-confirmed pneumonia was greater in children younger than 2 years (23.4% reduction) than among children 2 years or older (9.1% reduction). In South Africa, vaccination significantly reduced radiologically confirmed pneumonia in HIV-negative children. In the Gambia, vaccination reduced radiologically confirmed pneumonia by 37%, including a reduction of hospital admissions and all-cause mortality by 15 and 16%, respectively. This trial showed that vaccination could reduce deaths in children, and highlight the substantial role that pneumococcal infections play in deaths in young children in developing countries. Studies in the U.S., Israel and Europe have evaluated effects of PCVs on clinical or culture-proved pneumococcal otitis media. In Northern California, among those receiving PCV there was a 7.0% reduction in otitis media episodes, 9.3% reduction in frequent otitis media and a 20.1% reduction in ventilatory tube placement compared with children receiving control vaccine.2 In Finland, investigators evaluated 2 PCV formulations with different carrier proteins.

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