Epidemiology

S. pneumoniae is a transient member of the normal flora, colonizing the nasopharynx of up to 40% of healthy adults and children with no adverse effects. Children carry this pathogen in the nasopharynx asymptomatically for about 4-6 weeks, often several serotypes at a time. New serotypes are acquired approximately every 2 months. Serotypes 6, 14, 18, 19, and 23 are the most prevalent, accounting for 60-80% of infections depending on the area of the world. Pneumococcal infection accounts for more deaths than any other vaccine-preventable bacterial disease. Those most commonly at risk for pneumococcal infection are children between 6 months and 4 years of age and adults over 60 years of age. Virtually every child will experience pneumococcal otitis media before the age of 5 years. It is estimated that 25% of all community-acquired pneumonia is due to pneumococcus (1,000 per 100,000 inhabitants).

Until 2000, S. pneumoniae infections caused 100,000-135,000 hospitalizations for pneumonia, 6 million cases of otitis media, and 60,000 cases of invasive disease, including 3300 cases of meningitis. (CDC reported 60,000 cases of invasive pneumococcal disease in 1997, resulting in approximately 6,000 deaths.) The incidence of sterile-site infections haS shown geographic variation from 21 to 33 cases per 100,000 population. Disease figures are now changing due to conjugate vaccine introduction. In 2002, the rate of invasive disease was 13 cases per 100,000 in the United States. However, epidemics of disease have reappeared in settings such as chronic care facilities, military camps and day care centers, a situation not recognized since the pre-antibiotic era.

Also of concern, is the increased emergence of antibiotic resistance, especially in the past two decades. Multiple antibiotic resistant strains of S. pneumoniae that emerged in the early 1970s in Papua New Guinea and South Africa were thought to be a fluke, but multiple antibiotic resistance now covers the globe and has rapidly increased since 1995. Increases in penicillin resistance have been followed by resistance to cephalosporins and multidrug resistance. The incidence of resistance to penicillin increased from <0.02 in 1987 to 3% in 1994 to 30% in some communities in the United States and 80% in regions of some other countries in 1998. Resistance to other antibiotics has emerged simultaneously: 26% resistant to trimethoprim-sulfa, 9% resistant to cefotaxime, 30% resistant to macrolides, and 25% resistant to multiple drugs. Resistant organisms remain fully virulent but seem to have arisen in less than 10 serotypes. Serotypes 6A, 6B, 9V, 14, 19A and 23F are included in the vast majority of resistant strains.

Vaccines

Given the 90 different capsular types of pneumococci, a comprehensive vaccine based on polysaccharide alone is not yet feasible. Thus, vaccines based on a subgroup of highly prevalent types have been formulated. The number of serotypes in the vaccine has increased from four in 1945, to 14 in the 1970s, and finally to the current 23-valent formulation (25 mg of each of serotypes 1, 2, 3, 4, 5, 6B, 7F, 8, 9N, 9V, 10A, 11A, 12F, 14, 15B, 17F, 18C, 19A, 19F, 20, 22F, 23F, and 33F). These serotypes represent 85-90% of those that cause invasive disease, and the vaccine efficacy is estimated at 60% . However, underutilization of the vaccine is so extensive that the pneumococcus remains the most common infectious agent leading to hospitalization in all age groups. This is further complicated by the fact that polysaccharides are not immunogenic in children under the age of 2 years where a significant amount of disease occurs. Immunization is suggested for those at highest risk of infection, including those 65 years or older, and generally should be a single lifetime dose.

In the United States, a heptavalent pneumococcal conjugate vaccine (PCV7) has been recommended since 2000 for all children aged 2-23 months and for at-risk children aged 24-59 months. The four-dose series is given at 2, 4, 6 and 12-14 months of age. Protection is good against against invasive pneumococcal infections, especially septicemia and meningitis. However, children exposed to a serotype not contained in the vaccine are not afforded any protection. This limitation, and the ability of capsular-polysaccharide conjugate vaccines to promote the spread of non-covered serotypes, has led to research into vaccines that would provide species-wide protection.

For more Information on Streptococcus pneumoniae see

Streptococcus pneumoniae Disease - Technical Information from CDC

Streptococcus pneumoniae - Additional Information and Links from CDC