Interplay between maternal Tdap and infant pneumococcal vaccination in shaping infant pneumococcal vaccine serotype carriage

Background Tetanus, Diphtheria and acellular Pertussis (Tdap) vaccination during pregnancy blunts the infant humoral immune response following primary immunization with pneumococcal conjugate vaccines (PCVs). While this effect typically resolves after the booster dose for most vaccine serotypes, its impact on nasopharyngeal carriage of pneumococcal vaccine serotypes remains unclear. Methods A total of 3,298 nasopharyngeal swabs were collected from infants aged 6-30 months attending daycare centers in Belgium between 2018 and 2022, along with data on maternal Tdap vaccination status (clinicaltrials.gov identifier: NCT02888457). Streptococcus pneumoniae carriage and serotyping were assessed using culture-based methods (Quellung reaction) and molecular detection (LytA qPCR and serotype-specific qPCR). The association between Tdap vaccination during pregnancy and pneumococcal vaccine-related serotype carriage in infants was evaluated using logistic generalized estimating equation models. Results PCV13-related serotype carriage was significantly higher in offspring of Tdap-vaccinated mothers during pregnancy compared to those born to Tdap-unvaccinated mothers. In addition, children who received a PCV10 or mixed PCV10/PCV13 schedule had significantly higher PCV13-related serotype carriage compared to those immunized exclusively with PCV13. No significant differences were observed in individual PCV13-related serotype carriage, except for a significantly higher carriage of the PCV13-related serotype 6C in children of Tdap-vaccinated mothers. No significant difference was found for non-vaccine serotype carriage. Conclusions Tdap vaccination during pregnancy was associated with increased pneumococcal vaccine-related serotype carriage in infants, though the clinical significance remains uncertain. Future studies integrating vaccine serotype carriage data with protective pneumococcal antibody levels are needed to inform future maternal and infant vaccination strategies.