Invasive Pneumococcal Disease in Children: Changing Serotypes and Clinical Expression of Disease

Sir—In a recent article, Byington et al. [1] provided an interesting analysis of the temporal trends in the incidence of invasive pneumococcal disease (IPD) before and after the introduction of the heptavalent pneumococcal conjugate vaccine (PCV7). One of the most interesting findings they reported was the significant increase in the incidence of empyema from the prelicensure to the postlicensure period (10.3 vs. 14.3 cases/100,000 children, P =.013). However, the effects of the vaccine do not explain the increased incidence of empyema. Similar increases that clearly preceded the introduction of PCV7 have been reported in the last decade [2, 3]. Furthermore, shifts in the serogroups causing IPD had been observed just before the introduction of PCV7 [4], and the incidence of complicated community-acquired pneumonia has been reported to have decreased since the vaccine was introduced [5]. These changes could be related, at least in part, to the emergence of serogroups that were identified a long time ago as causes of IPD and that reappeared before the introduction of PCV7. Serogroup 1 has been associated with such changes; the percentage of cases of IPD with pleural involvement due to serogroup 1 increased from 24% [6] to 50% [7] between 1993 and 2000 in the United States. It could be that clonal spread of serogroup 1, as has been described in Sweden [8], in conjunction with the phenomenon of antibiotic tolerance [9], explains the reemergence of this serogroup in western societies and, consequently, the increasing number of parapneumonic complications. Also, as reported by Byington et al. [1], the incidence of IPD caused by serogroup 3 has significantly increased between the pre- and postlicensure periods.