Vaccination of Icelandic children with the 10-valent pneumococcal vaccine leads to a significant herd effect among adults in Iceland.
Quirk SJ., Haraldsson G., Hjálmarsdóttir MÁ., van Tonder AJ., Hrafnkelsson B., Bentley SD., Haraldsson Á., Erlendsdóttir H., Brueggemann AB., Kristinsson KG.
The introduction of pneumococcal conjugate vaccines (PCV) into childhood vaccination programs has reduced carriage of vaccine serotypes and pneumococcal disease. The 10-valent PCV was introduced in Iceland in 2011. The aim of this study was to determine PCV impact on the prevalence of serotypes, genetic lineages and antimicrobial-resistant pneumococci isolated from the lower respiratory tract (LRT) of adults.Pneumococci isolated during 2009-2017 at the Landspitali University Hospital, were included (n=797). The hospital serves almost three-quarters of the Icelandic population. Isolates were serotyped and tested for antimicrobial susceptibility, and the genome of every other isolate from 2009-2014 was sequenced (n=275). Serotypes and multilocus sequence types (STs) were extracted from the genome data. Three study periods were defined: 2009-2011 (PreVac), 2012-2014 (PostVac-I) and 2015-2017 (PostVac-II).The total number of isolates and vaccine-type (VT) pneumococci decreased from PreVac to PostVac-II (n=314 vs n=230; p=0.002 and n=170 vs n=33; p<0.001, respectively), but nonvaccine-type (NVT) pneumococci increased among adults 18-64 years old (n=56 vs n=114; p=0.008). Serotype 19F decreased in the PostVac-II period: these isolates were all multidrug-resistant (MDR) and were members of the Taiwan19F-14 PMEN lineage. Serotype 6A decreased among adults ≥65 years old PostVac-II (p=0.037), while serotype 6C increased (p=0.021) and most serotype 6C isolates were MDR. Non-encapsulated S. pneumoniae (NESp) increased among adults 18-64 years old PostVac-II and the majority were MDR (p=0.028).An overall reduction in the number of LRT samples, pneumococcal-positive cultures, and significant changes in the serotype distribution became evident within four years thereby demonstrating a significant herd effect.