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purified pneumococcal capsular polysaccharides have shown that they are effective in stimulating type-specific immunity. Recovery from natural infection with a given pneumococcal type usually is followed by lifelong immunity to reinfection with that type.

Type-specific capsular antibodies of the IgG class may be transferred transplacentally from mother to fetus during pregnancy and provide some protection against infection with homologous types during the first 6 months of life. Maximum attack rates of pneumococcal infection, including otitis media and bacteremia, occur between the ages of 6 months and 2 years. Initial immunologic responses to vaccination with pneumococcal capsular polysaccharides in infancy are predominantly of the IgM class, and the protection afforded by such antibodies is of limited duration, usually not exceeding 6 months. The ability to respond to different pneumococcal capsular polysaccharides develops at different ages; immunity to type 3 capsular polysaccharide is observed as early as age 6 months. Responsiveness to the serotypes responsible for the majority of infections in early life (types 6A, 6B, 14, 19F, 19A, and 23F) may be delayed until the age of 4 or later. Preliminary studies of type 6A pneumoccocal capsular polysaccharide coupled to tetanus toxoid suggest that responsiveness to the conjugated antigen occurs at a significantly earlier age and induces IgG as well as IgM antibodies.

Limited data on the responsiveness of the elderly to parenteral administration of pneumococcal capsular polysaccharides suggest that, although they respond somewhat less vigorously than young adults, their responses are significant and result in protection against infection. Patients who have acquired immunologic deficiencies, such as those resulting from lymphocytic malignancies (e.g., multiple myeloma) respond feebly or not at all to the current polyvalent pneumococcal vaccine.

DISTRIBUTION OF DISEASE

Geographic Distribution

Pneumococcal pneumonia, otitis media, and meningitis occur throughout the world and have been found to be endemic wherever they have been sought. Higher than average attack rates have been observed in populations living in depressed socioeconomic conditions and in association with movement from rural to urban environments. Epidemics also may occur under conditions in which immunologically naive adults are congregated in industrial or military barracks, the so-called “recruit disease” phenomenon.

Disease Burden Estimates

The burden of pneumococcal infection is large, but it is difficult to obtain accurate data on the incidence of pneumococcal pneumonia and pneumococcal otitis media for a variety of reasons. The incidence of



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