Back to the future? Drastic drop in serotype 19A carriage in daycare centers within two years after a second switch to PCV13 in Belgium.

Pneumococcal conjugate vaccines (PCVs) reduce Streptococcus pneumoniae infection and carriage. After switching from PCV13 to PCV10 in 2015-2016, Belgium switched back to PCV13 in 2019. Building on our systematic monitoring of childhood nasopharyngeal carriage since 2016, here, we analyze the serotypes of S. pneumoniae and other pathogens in children attending daycare centers (DCCs) from 2018 to 2021. From the period of 2018-2019 to 2020-2021, we included a total of 2,741 nasopharyngeal swabs collected from children aged 6 to 30 months. We identified S. pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, and Staphylococcus aureus and conducted serotyping and antimicrobial susceptibility assessments of S. pneumoniae strains using culture methods and real-time PCR. S. pneumoniae carriage was frequent and quite stable over the three study years. H. influenzae and M. catarrhalis were more frequently carried than S. pneumoniae. Frequency of all PCV13-serotypes together among S. pneumoniae carriers decreased significantly from 19.4% in 2018-2019 to 9.9% in 2020-2021 (p < .001), largely due to the decreased serotype 19A carriage. Resistance of pneumococcal strains to penicillin increased significantly over the three study years. Two years after the second switch to PCV13 in 2019, pneumococcal serotype 19A carriage decreased again significantly in Belgian children attending daycare centers.