Evolving Picture of Invasive Pneumococcal Disease in Massachusetts Children: A Comparison of Disease in 2007-2009 With Earlier Periods

Background: As expected, the heptavalent pneumococcal conjugate vaccine (PCV7) had a significant impact on invasive pneumococcal disease (IPD) in children. In addition to the substantial decline in IPD, increased disease due to nonvaccine serotypes and a changing clinical presentation emerged. The objective of this study is to describe these trends in IPD in the late PCV7-era. Methods: We report on continued, prospective, population-based surveillance of childhood IPD in Massachusetts children during the period 2007 to 2009 and make comparisons with the earlier 2001 to 2006 PCV7-era. Demographic and clinical data were collected for all cases. Streptococcus pneumoniae isolates from normally sterile sites were serotyped and further evaluated using antimicrobial susceptibility testing, multilocus sequence typing and eBURST analysis. IPD incidence rates are calculated by age, year and serotype. Results: There were 326 cases of IPD between 2007 and 2009 in children < 18 years of age. Overall IPD incidence rate was 7.5 cases per 100,000 population and was not statistically different from the observed incidence in 2001 to 2006 (P > 0.05). As compared with the earlier period, the proportion of bacteremic pneumonia among all IPD cases was almost 3-fold greater in 2009 to 2010 (P < 0.01). PCV7 serotypes accounted for 7%, whereas the 13-valent pneumococcal conjugate vaccine serotypes accounted for 77% of all cases between 2007 and 2009. IPD due to serotypes 19A and 7F increased, and 19A and 7F were isolated in 41% and 20% of all IPD cases in the same period, respectively. Serotype 19A also comprised a majority of the penicillin-and ceftriaxone-resistant isolates. Analysis of multilocus sequence typing data showed a significant increase in ST191, ST695 and ST320 and a significant decrease in ST199 and ST180. Conclusions: The reduction in IPD after introduction of PCV7 persists in Massachusetts children; however, serotypes causing IPD have changed significantly in the last decade. Continued surveillance is necessary to determine the impact of 13-valent pneumococcal conjugate vaccine, as well as track potential changes in disease incidence and character due to non-13-valent pneumococcal conjugate vaccine serotypes.