Version 1: Increase in prevalence of Streptococcus pneumoniae serogroup 24 in children upon introducing 13-valent pneumococcal conjugate vaccine in Japan

After introducing the 13-valent pneumococcal conjugate vaccine (PCV13) in children, a change in the prevalence of different Streptococcus pneumoniae serotypes that cause invasive pneumococcal diseases (IPDs) has been observed; the prevalence of vaccine serotypes has decreased and that of non-vaccine serotypes has increased. Nowadays, serogroup 24 has become one of the major non-vaccine serotypes causing IPDs in children in Japan. The aim of this study is to characterize clinical and genomic features of the S. pneumoniae serogroup 24 strains isolated from sterile body sites in Japanese children. Serotyping, multi-locus sequence typing and genomic analysis of capsular polysaccharides were performed on 61 strains of serogroup 24 from 2015 to 2021. Among the 61 strains, 36, 23, and 2 strains belonged to serotypes 24F, 24B, and 24C, respectively. Serotypes 24F ST 2572 and 24B ST 2572 were the major serotypes and sequence types observed from 2015 to 2019. On the contrary, 24F ST 162 and 24B ST 2754 were the two major serotypes and sequence types observed after 2020. Two strains of serotype 24C were detected for the first time in Japan. Analysis of the sequence of the abpA gene, which plays a role in the synthesis of capsular polysaccharides in S. pneumoniae, was performed to distinguish between different strains of serogroup 24. After the introduction of PCV13 in Japan, serogroup 24 has become one of the most prevalent non-vaccine serotypes causing IPDs in children. This serogroup has not targeted by the next generation pneumococcal conjugate vaccines. Monitoring of S. pneumoniae serogroup 24 that causes IPDs in children is essential.