Demonstration of the herd effect in adults after the implementation of pneumococcal vaccination with PCV13 in children.

The purpose of this investigation was to describe the evolution of serotypes and antibiotic susceptibility of Streptococcus pneumoniae strains isolated from both adults and children from the same population area with invasive pneumococcal disease (IPD) or acute otitis media (AOM), 5 years after the introduction of the 13-valent pneumococcal conjugate vaccine (PCV13). From 2009 to 2015, 839 strains of S. pneumoniae strains were collected (481 from adults and 358 from children). Serotyping by latex antisera and molecular methods was performed. Antimicrobial susceptibility was tested. Compared to 2009, the total number of strains isolated in 2015 decreased in children (263 vs. 53, respectively) and in adults (220 vs. 131, respectively). Serotype coverage of PCV13 for IPD decreased significantly in adults from 67.7% (149/220) to 25.2% (33/131) and in children from 75.1% (61/81) to 18.5% (5/27). Especially, serotypes 1, 7F and 19A decreased significantly in children, while serotypes 7F and 19A decreased significantly in adults. PCV13 serotypes involved in AOM decreased significantly over the 5-year period, from 85.7% (156/182) to 38.5% (10/26), and were more susceptible to penicillin, amoxicillin and cefotaxime, p < 0.05. Serotypes 8, 9N and 10A seemed to emerge in adults, whereas any serotype prevalence was observed in children. Between 2009 and 2015, the introduction of PCV13 has resulted in a significant decrease of the number of S. pneumoniae strains isolated from IPD in children as in adults. It highlights a strong herd effect of vaccination in adults.