Background: The 13-valent pneumococcal conjugate vaccine (PCV13) has been widely implemented to reduce the burden of Streptococcus pneumoniae-related diseases in children. However, serotype replacement poses a challenge to its long-term efficacy, particularly for non-invasive respiratory infections. This study evaluates the clinical effectiveness of PCV13 against acute respiratory infections (ARIs) in children in Saint Petersburg, Russia, amidst serotype replacement.

Methods: A retrospective cohort study was conducted, analyzing data from 455 children (2014 birth cohort, n=134; 2022 birth cohort, n=321). Data on ARI, otitis media, and pneumonia incidence, PCV13 vaccination status, and risk factors (e.g., daycare attendance, allergic, and neurological history) were collected from medical records. Statistical analysis included Mann-Whitney, Wilcoxon, and Friedman tests, with multifactorial logistic regression adjusting for age and comorbidities (R 4.2.2, p<0.05).

Results: PCV13 vaccination significantly reduced ARI incidence in the first year of life (1.12 ± 1.43 in vaccinated vs. 1.58 ± 1.38 in unvaccinated; p=0.030), but had no significant effect on otitis media or pneumonia (p>0.05). Daycare attendance increased ARI incidence 2.1-fold (p<0.001). A significant increase in ARI incidence was observed in vaccinated children from the 2022 cohort compared to the 2014 cohort (2.02 ± 1.83 vs. 1.12 ± 1.43; p<0.001), suggesting serotype replacement.

Conclusions: PCV13 maintains partial efficacy against ARIs in early childhood, but its effectiveness is diminished by serotype replacement, as evidenced by increased ARI incidence in the 2022 cohort. These findings underscore the need for regular serotype surveillance, consideration of broader-valency vaccines (e.g., PCV15/20), and enhanced preventive measures for children in daycare settings.

Keywords: Pneumococcal conjugate vaccine, serotype replacement, acute respiratory infections, vaccination, children