Direct and indirect effects of PCV13 on nasopharyngeal carriage of PCV13 unique pneumococcal serotypes in Massachusetts' children.

Background: The direct impact of 13-valent pneumococcal conjugate vaccine (PCV13) on colonization with unique PCV13 serotypes and the uptake of vaccine necessary to create indirect protection in nonimmunized children were assessed.

Methods: Carriage surveillance among children <60 months began in July 2010 at a pediatric practice in Boston, MA. Children had nasopharyngeal cultures and parents completed questionnaires detailing demographics and health status. Concurrently, we monitored uptake of PCV13 in children in the community. Children were classified as "presumed immune" or "presumed nonimmune" based on age and PCV13 immunizations received. We assessed trends using adjusted prevalence rates calculated within rolling, 25-week, consecutive intervals.

Results: Between July 2010 and June 2012, 1050 S. pneumoniae isolates were recovered from 1042 children. Eighty-nine isolates (8.5%) were 1 of 6 unique PCV13 serotypes. The expected fall/winter peak in PCV13 carriage was observed in nonimmune children, but was blunted in immune children. There was a 74% reduction in PCV13 colonization in immune compared with nonimmune children. We document a 50% or more decline in the PCV13 carriage in nonimmune children, at the time when the approximately 75% or more of the community children had received PCV13 and were considered immune. During the study, the difference in PCV13 serotype colonization prevalence in nonimmune and immune children disappeared. No evidence of replacement has been observed to date.

Conclusions: The direct impact of PCV13 on colonization was demonstrated. Evidence of indirect protection in unimmunized (nonimmune) children was observed as vaccine uptake reached 75% in the target community.