Effect of vaccination on pneumococci isolated from the nasopharynx of healthy children and the middle ear of children with otitis media in Iceland

Vaccination with pneumococcal conjugate vaccines (PCVs) disrupts the pneumococcal population. Our aim was to determine the impact of the 10-valent PCV on the serotypes, genetic lineages and antimicrobial susceptibility of pneumococci isolated from children in Iceland. Pneumococci were collected between 2009-2017 from the nasopharynx of healthy children attending 15 day-care centres and from the middle ear (ME) of children with acute otitis media from the greater Reykjavik capital area. Isolates were serotyped and tested for antimicrobial susceptibility. Whole genome sequencing (WGS) was performed on alternate isolates from 2009-2014 and serotypes and multilocus sequence types (STs) were extracted from the WGS data. Two study periods were defined: 2009-2011 (PreVac) and 2012-2017 (PostVac). The overall nasopharyngeal carriage rate was similar between the two periods (67.3% PreVac and 61.5% PostVac, p=0.090). Vaccine-type (VT) pneumococci decreased and nonvaccine-type (NVT) pneumococci (serotypes 6C, 15A, 15B/C, 21, 22F, 23A, 23B, 35F, 35B) significantly increased post-PCV introduction in different age strata. The total number of pneumococci recovered from ME samples significantly decreased, as did the proportion that were VTs, although NVT pneumococci (6C, 15B/C, 23A and 23B) increased significantly. Most serotype 6C pneumococci were multidrug-resistant (MDR). Serotype 19F was the predominant serotype associated with ME and it significantly decreased post-PCV introduction: these isolates were predominantly MDR and of the Taiwan 19F -14 PMEN lineage. Overall, the nasopharyngeal carriage rate remained constant and the number of ME-associated pneumococci decreased significantly post-PCV introduction; however, there was a concomitant and statistically significant shift from VTs to NVTs in both collections of pneumococci.