Background: In clinical trials, the seven-valent pneumococcal vaccine (Prevnar) was effective against invasive pneumococcal disease and pneumonia and moderately effective against otitis media. The vaccine was recommended for all children younger than two years and for high-risk children two to four years of age. Between 2001 and 2004, several vaccination schedules were used, and problems with vaccine supply resulted in some children not receiving as many vaccinations as planned or receiving delayed booster doses. Whitney and colleagues assessed the overall effectiveness of the seven-valent pneumococcal vaccine with attention to the influence of dose schedule in preventing pneumococcal disease.

The Study: The authors used the national Active Bacterial Core Surveillance system to identify cases of invasive pneumococcal disease in children four years and younger. Cases were identified by bacterial isolates. All isolates were typed and tested for antimicrobial sensitivity. Case children were compared with controls matched by age and location of birth. Data collected from case and control children included number of siblings, attendance at day care, exposure to cigarette smoke, history of breastfeeding, and history of chronic medical conditions. Participants' parents or guardians gave permission for medical information to be accessed, specifically immunization records and medical histories.

Results: Of the 1,267 children with invasive pneumococcal disease who were identified, 782 were included in the study. Exclusions resulted from parents refusing permission to participate, incomplete immunization history or medical data, and failure to confirm a pneumococcal isolate. Enrolled children were more likely to have been white and less likely to have died than nonenrolled children. The median age of the enrolled children was 21 months; 40 percent were girls, and 59 percent had been treated without hospital admission. The most common manifestation of infection was bacteremia (51 percent); this was followed by pneumonia (30 percent) and meningitis (8 percent). A vaccine serotype was identified in 45 percent of cases, with types 14 and 19F being the most common.

Overall, the vaccine effectiveness against the targeted seven serotypes was 96 percent in healthy children and 81 percent in children with comorbidities. The effectiveness was similar for the different clinical manifestations of pneumococcal infection and ranged from 100 percent for serotype 9V to 87 percent for serotype 19F. Protection against the vaccine-related serotypes 6A and 19A was 76 and 26 percent, respectively. For all pneumococcal serotypes, effectiveness was 71 percent in healthy children and 77 percent in those with comorbidities. One dose of pneumococcal vaccine given at or before seven months of age appeared to be protective against targeted serotypes for up to six months. In comparing the most commonly used vaccination schedules, the addition of a booster dose at 12 to 16 months of age to the schedule of three doses given before seven months of age resulted in a significantly reduced risk of disease.

Conclusion: The authors conclude that the seven-valent pneumococcal vaccine is highly protective against disease attributable to its target serotypes. Although a single vaccination before six months of age does not appear protective, several other schedules provided good protection. The most effective schedule (three doses before seven months of age plus a booster at 12 to 16 months) may be too complex and expensive for some populations. They note that vaccination has promoted a rise in herd immunity in the United States, with the number of cases falling from 17,240 in 1998 and 1999 to 4,454 in 2003.