CC180 clade dynamics do not universally explain Streptococcus pneumoniae serotype 3 persistence post-vaccine: a...

Clonal complex 180 (CC180) is currently the major clone of serotype 3 Streptococcus pneumoniae (Spn) causing disease among children and adults worldwide. The 13-valent pneumococcal conjugate vaccine (PCV13) does not have significant efficacy against serotype 3 despite polysaccharide inclusion in the vaccine. It was hypothesized that PCV13 may effectively control Clade I of CC180 but that Clades III and IV are resistant, provoking a population shift that enables serotype 3 persistence. This has been observed in the United States, England, and Wales but not Spain. We tested this hypothesis further utilizing a dataset from Portugal to conduct our population genomics and molecular epidemiology comparative study.

We performed whole-genome sequencing (WGS) of 501 serotype 3 strains from Portugal isolated from patients with pneumococcal infections between 1999 and 2020. The draft genomes underwent phylogenetic analyses, pangenome profiling, and a genome-wide association study (GWAS). We also completed antibiotic susceptibility testing and compiled over 2600 serotype 3 multilocus sequence type 180 (MLST180) WGSs to perform global comparative genomics.

Relative to strains from all other lineages, CC180 Clades I, II, III, IV, and VI strains trend towards a decreased association with invasive disease cases compared to non-invasive pneumonia cases (binomial logistic regression, odds ratio or OR = 0.59, 95% confidence interval or CI = [0.34, 0.98], P = 0.046) and adult patients compared to paediatric patients (binomial logistic regression, OR = 0.34, 95% CI = [0.098, 0.92], P = 0.054). The serotype 3 CCs shifted post-PCV13 such that Clades I-VI comprise the majority of post-PCV13 lineages (binomial logistic regression, OR = 7.33, 95% CI = 4.36, 12.80, P < 0.0001), with Clade I representing 54% (220/404) of all post-PCV13 strains. As observed elsewhere, Clade I strains from Portugal are largely antibiotic-sensitive and carry the ΦOXC141 prophage. However, strains from Portugal and Spain, where Clade I remains dominant post-PCV13, have larger pangenomes and are associated with the presence of two genes encoding hypothetical proteins.

Clade I became dominant in Portugal post-PCV13, despite the burden of the prophage and antibiotic sensitivity. The additional accessory genome content may mitigate these fitness costs. Regional differences in Clade I prevalence and pangenome heterogeneity suggest that clade dynamics is not a generalizable approach to understanding serotype 3 vaccine escape.

National Institute of Child Health and Human Development, Pfizer, and Merck Sharp & Dohme.