Pneumococcal vaccine

Streptococcus pneumoniae, or pneumococcus, is an encapsulated gram-positive coccus. Pneumococci are a major cause of common upper respiratory infectio ns and life-threatening, invasive infections in children, adolescents, and adults in the United States and worldwide resulting in much morbidity and m ortality. Pneumococci cause an estimated 3000 cases of meningitis, 50,000 c ases of bacteremia, and 500,000 cases of pneumonia in the United States eac h year.(12) Cases of bacterial pneumonia and meningitis account for more th an 500,000 hospitalizations and more than 40,000 deaths in the United State s each year. In the United States, pneumococci are the most common cause of bacterial meningitis in children 1 to 23 months of age(54) and pyogenic pn eumonia in pediatric patients beyond the neonatal period. The spectrum of i nvasive infections caused by pneumococci also includes septic arthritis, os teomyelitis, bacteremic cellulitis, primary peritonitis, and occult bactere mia in highly febrile infants and young children, and sepsis that may be ra pidly fatal in immunocompromised hosts, such as those with impaired splenic function. S. pneumonine is the leading cause of the common bacterial upper respiratory tract infections, acute otitis media acid sinusitis. Seven mil lion cases of acute otitis media are diagnosed each year in the United Stat es, and 62% of US infants have an episode of otitis media during the first year of life.(12) The pneumococcal cell may be enveloped by 1 of at least 90 chemically disti nct polysaccharide capsules. Landmark studies performed during the first ha lf of the twentieth century demonstrated that protective immunity was confe rred by serum from rabbits injected with heat-killed pneumococci, that the protective activity of the serum is pneumococcal type specific, and that po lysaccharide capsular is the major virulence factor and is the target of ty pe-specific immunity (i.e., antibody). These findings led to capsular typin g of pneumococcal isolates using antiserum prepared in animals by the Quell ung reaction, which is the capsular swelling noted microscopically after in cubation of the isolate with type-specific antiserum. In early studies, kil led bacteria and later purified capsular polysaccharides were studied as va ccines. In addition, capsular type-specific horse antiserum was used for th e treatment of pneumococcal infection in humans. Thus, by the mid-1900s, re searchers had established that capsule serves as a major virulence determin ant, allowing the bacterium to resist phagocytosis and killing in the absen ce of anticapsular antibodies. They had also established that serotype-spec ific, anticapsular antibodies can be generated by immunization and play a m ajor role in type-specific protection against pneumococcal disease.