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

Abstract

After introducing the 13-valent pneumococcal conjugate vaccine (PCV13) for 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. Currently, serogroup 24 has become one of the major non-vaccine serotypes causing IPDs in children in Japan. The aim of this study was to characterize clinical and genomic features of S. pneumoniae serogroup 24 strains isolated from sterile body sites in Japanese children. Serotyping, multi-locus sequence typing and genomic analysis of capsular polysaccharides of 61 strains of serogroup 24 were performed from 2015 to 2021. Among the 61 strains, 36, 23 and two belonged to serotypes 24F, 24B and 24C, respectively. The 24F sequence type (ST) 2572 and 24B ST 2572 were the major serotypes and sequence types observed from 2015 to 2019. By contrast, 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. Sequence analysis of the abpA gene, which plays a role in the synthesis of capsular polysaccharides in S. pneumoniae , was performed to distinguish 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 been targeted in the next-generation pneumococcal conjugate vaccines. Therefore, monitoring of S. pneumoniae serogroup 24 that causes IPDs in children is essential.

Keywords: Streptococcus pneumoniae; abpAgene; sequence type; serogroup 24.

Fig. 1.

Annual change in the prevalence of different sequence types of Streptococcus pneumoniae serogroup 24 isolated from children with invasive pneumococcal disease (IPD). Among the 61 isolated strains, 36, 23 and two belonged to serotypes 24F, 24B and 24C, respectively. The number in parentheses is the total IPD cases each year. The y-axis gives the number of serogroup 24 strains.

Fig. 2.

Sequence type (ST), serotype and strain number of Streptococcus pneumoniae and multiple alignments of amino acids of the protein encoded by abpA. ST 2572 was detected in serotypes 24F, 24B and 24C; ST 162 in serotypes 24F and 24C; ST 5496 in all serotype 24F strains; and ST 2754 in all serotype 24B strains. Isolated strains with more than one strain listed had the same amino acid. Amino acid alignment was performed using the amino acid sequence of ST 2572, with serotype 24F as a reference. Amino acids identical to those of the reference sequence are indicated with dots, and sequences differing from the reference are indicated with uppercase letters. Numbers above the alignment indicate the position of the amino acid. Asterisks indicate stop codons.