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Pneumococcal conjugate vaccination for adults: why it's important for children.

The subject of pneumococcal conjugate vaccine was comprehensively reviewed in these pages last year. This issue of the journal reports the results of the Kaiser-Northern California clinical trial of pneumococcal conjugate vaccine in children. Given these wonderful results, few can doubt that implementation of conjugate vaccination in the years ahead will have a major impact on the health care of children throughout the world. One can hardly imagine a better way to start the new millennium. An excellent study such as this inevitably invites questions. An internist would like to know what the implications of these trial results are for the prevention of pneumococcal infections in adults. A pediatrician, in turn, might ask why conjugate vaccination of adults is important for children. Answers to both questions require an understanding of the effectiveness and limitations of 23-valent pneumococcal polysaccharide vaccination, the status of conjugate vaccine studies in adults and the global importance of pneumococcal disease and its prevention in persons of all ages. Pneumococcal polysaccharide vaccine has been available for use in persons $2 years in age for .20 years. Retrospective studies have firmly established its clinical effectiveness in preventing invasive pneumococcal disease in older adults. Moreover the cost effectiveness of doing so has been demonstrated not only for the United States but also for several countries in western Europe. For preventing nonbacteremic pneumococcal pneumonia in older adults, prospective clinical trials have been inconclusive, but a recent retrospective study has shown that vaccination reduces rates of hospitalization for pneumonia and all-cause mortality in persons with chronic lung disease. As a result of better understanding, the use of the vaccine, largely confined to the United States and Iceland before 1994, has recently increased in many developed countries. Although the benefits of the 23-valent polysaccharide vaccine are undeniable, it has several limitations, some of which are summarized in Table 1. In the absence of immunologic memory, antibody levels to important serotypes decline to prevaccination values within 3 to 7 years. This helps to explain the low levels of clinical protection against several of these serotypes and the decline in overall clinical protection observed during the same period. Fortunately most older individuals mount antibody responses to the 23 vaccine serotypes that are comparable to those of younger adults when assessed not only by enzyme-linked immunosorbent assays but also by avidity and opsonophagocytic assays. However, these responses are not uniform across all 23 serotypes, and some older individuals may respond poorly to most serotypes in the vaccine. 11 Recent studies have shown that antibody avidity, not the level of antibody itself, is better correlated with opsonophagocytic activity in vitro 12 and that opsonophagocytic activity is better correlated with protection against experimental challenge infec-