Vaccine-Induced Immunity to Pneumococcal Infection

A consensus of the importance of the bacterial capsule for protective immunity has prevailed from very early on, based on the serotype specificity of the first whole-cell vaccines and soon after that on the protective immunity observed after vaccination with the purified polysaccharide. The evidence of serotype-specific protection against bacteremia in immunocompetent adults, starting approximately 2 weeks after a single dose of the polysaccharide vaccine, is unequivocal based on both clinical trials before vaccine licensure and the subsequent experience with the wide use of the polysaccharide vaccine. Pneumococcal pneumonia was believed to be a major cause of illness and even death in these populations. However, a vaccine effect could not be seen in either of the trials with respect to overall or serotype-specific pneumococcal pneumonia or pneumonia-associated deaths. In view of the importance of pneumonia for children in developing countries, it is regrettable that further studies to confirm or refute these findings have not been conducted in the face of a lack of funding and an expectation of an improved vaccine. In recent studies in Finland almost every child has been shown to have had at least one episode, and many several, of otitis media before the age of 2 years, with pneumococci present in the purulent middle ear fluid in 30% of these cases. The major study to test the protective efficacy of the 7-valent conjugate vaccine was carried out in California, within the framework of the large health insurance plan of Kaiser Permanente.