Serotype prevalence of occult pneumococcal bacteremia

The licensure and use of a pneumococcal conjugate vaccine that is immunogenic in children who are younger than 2 years may affect the epidemiology of occult bacteremia. This study was conducted to determine the serotype prevalence of Streptococcus pneumoniae isolates from children with occult bacteremia and to document the proportion that would be covered by the recently licensed heptavalent pneumococcal conjugate vaccine.

A cohort of 5901 children who were 2 to 24 months of age and had a temperature of >/=39.0 degrees C evaluated with a blood culture at an urban tertiary care children's hospital emergency department was studied to determine the prevalence of S pneumoniae serotypes. Patients were excluded if their immune system was suppressed, they had a diagnosis of a focal infection, they were evaluated by lumbar puncture, they were admitted to the hospital, or they died during initial evaluation. Blood cultures were inoculated into pediatric blood culture bottles and processed using an automated carbon dioxide monitoring system. All pneumococcal isolates were serotyped on the basis of capsular swelling with type-specific antisera (Quellung reaction).

The study population consisted of 5901 patients. The overall rate of occult bacteremia was 1.9% (95% confidence interval [CI]: 1.5-2.3). S pneumoniae accounted for 92 of 111 isolates (82.9%; 95% CI: 74.6-89.4) in children with occult bacteremia. Eight pneumococcal serotypes were represented: 6A (2%), 9V (6%), 19F (6%), 18C (8%), 4 (9%), 6B (13%), 23F (15%), and 14 (42%). Serotypes 14, 6B, and 23F accounted for 69.3% (95% CI: 58.6-78.7) of typed isolates. In the cohort, 97.7% (95% CI: 92-99.7) of isolated serotypes are represented in the newly licensed heptavalent pneumococcal conjugate vaccine. The single isolated serotype that would not have been covered by the currently licensed heptavalent pneumococcal conjugate vaccine was 6A.

S pneumoniae accounts for the vast majority of bacterial pathogens in children with occult bacteremia. As indicated by the results of this study, the heptavalent pneumococcal conjugate vaccine may prevent the majority of occult pneumococcal bacteremia episodes. The 2 cases of bacteremia with a serotype that would not have been included in the vaccine both were due to serotype 6A. It has been noted that there is potential nonvaccine serotype and subgroup cross-protection (6A from 6B) afforded to children who are immunized with the heptavalent vaccine. The high potential efficacy of the heptavalent pneumococcal conjugate vaccine for strains that cause occult bacteremia in our population may have a profound effect on the treatment of children with fever without a source. There has been an alarming and rapid emergence of antibiotic-resistant pneumococcal strains. Less pressure to use broad-spectrum antibiotics, which in turn causes further antibiotic resistance, should result. Laboratory testing and hospitalization also should be reduced. The prevalence rates determined by this study may be used as baseline data for comparison of serotype rates of occult pneumococcal bacteremia after widespread use of the heptavalent vaccine.