A brief history of pneumococcal vaccines.

Attempts to control pneumococcal infection by vaccination, undertaken initially in 1911, have gone through 3 phases during the subsequent 8 decades. Initially, vaccines of killed pneumococcal cells prepared in a variety of ways were used in epidemic settings with inconclusive results, although administered to approximately 1 million recipients. The discovery that adults injected with small amounts of purified capsular polysaccharide developed antibodies to the homologous capsular type led to the trial of a tetravalent vaccine that showed conclusively its ability to prevent infection by the types represented in it. With the advent of penicillin and other effective antipneumococcal drugs, interest in prophylaxis waned. Interest in vaccination was revived only after demonstration that some segments of the population remained at high risk of death if infected and after the emergence of multidrug-resistant pneumococci. Infants and young children, among whom the incidence of pneumococcal infection is high, respond poorly to purified bacterial polysaccharides but develop satisfactory responses to bacterial polysaccharides when these are linked chemically to a protein. The early results of trials with such polysaccharide protein conjugate vaccines give promise that control of a significant portion of pneumococcal infection in the paediatric population will soon be feasible.