A comprehensive analysis of serotype-specific invasive capacity, clinical presentations, and mortality trends of invasive pneumococcal disease.

Background: Pneumococcal conjugate vaccines (PCV) reduced invasive disease, but the overall prevalence of pneumococcal nasopharyngeal colonization among children has not changed significantly. Our knowledge of which serotypes, once colonized, hold a higher likelihood to cause invasive disease is limited.

Methods: Serotype-specific invasive capacity (IC) of Streptococcus pneumoniae was estimated using an enhanced population-based invasive pneumococcal disease (IPD) surveillance in children <7 years of age in Massachusetts and surveillance of nasopharyngeal (NP) colonization in selected Massachusetts communities in corresponding respiratory seasons. Serotype-specific IC was calculated by dividing the annual incidence of IPD by the carriage prevalence for each serotype. Serotype-specific relationship between NP carriage and IPD was evaluated by year, age group (<24 months vs 24-84 months), pre-PCV13 (2003/04, 2006/07, 2008/09) vs post-PCV13 (2010/11, 2013/14, 2015/2016) periods, clinical presentation, and outcome.

Results: A total of 293 IPD and 1602 NP isolates were included in the analysis. Most common IPD serotypes were 19A (34.1 %), 7F (9.2 %), 15 BC (8.9 %), 3 (5.8 %), and 22F (4.8 %). Serotypes 18C, 38, 7F, 19A, 3, 22F, and 33F displayed a higher propensity to cause IPD once colonizing the nasopharynx compared to 11A, 35B, 6C, and 21. Serotype-specific IC was generally lower in children older than 24 months. During the study period, we observed shifts in the dominant serotypes in relation to IC as well as changes between pre- to post-PCV13 era. Except for serotypes 14, 6A, 7F, 11A, 23A, 20, 35F, 7C, 6C and 15F all serotypes presented primarily as bacteremia. Pneumonia was attributed to serotypes 14 and 20; serotypes 35B, 23B, and 11A were responsible for the highest percentage of deaths.

Conclusion: This study highlights the need for continued serotype-specific surveillance to better understand the disease potential of emerging serotypes and to guide optimal vaccination strategies.