High effectiveness of infant vaccination against pertussis in 1- and 2-year-olds, the Netherlands, 2013-2024.

INTRODUCTION With still high circulation of pertussis and risk for severe disease in young children, insight into vaccine effectiveness (VE) of infant pertussis vaccination is important. In the Netherlands, NIP period 1 corresponds to the period during which the DT3aP-IPV-Hib-HBV vaccine and the 3 + 1 dose schedule were used, and NIP period 2 to the period during which the DT5aP-IPV-Hib-HBV vaccine was used, maternal Tdap vaccination was implemented, and both the 3 + 1 and 2 + 1 dose schedules were used. We estimated the VE of full infant vaccination against pertussis.

Methods

We conducted a case-control study including all laboratory-confirmed pertussis cases aged 12-35 months, notified between 2013 and 2024, each matched to four controls by year and month of birth. Conditional logistic regression including age group and NIP period as interaction terms was used to estimate stratum-specific odds ratios (ORs) with 95% confidence intervals (CIs). We compared these estimates to those obtained by the screening method, applying logistic regression with the log-odds of the national vaccination coverage in the corresponding birth cohorts of cases as an offset. VE was calculated as (1-OR)*100 for both methods.

Results

We included 1156 cases and 4624 controls. Among cases, 27% were fully vaccinated, 4% partly vaccinated and 69% unvaccinated; among controls these percentages were 89%, 6% and 5% respectively. Overall VE of the case-control study was 98.2% (95% CI: 97.6-98.6). VE decreased with increasing time since vaccination (p < 0.001) and this was slightly more pronounced for NIP period 1 compared to period 2 (p < 0.001). Estimates using the screening method were slightly lower, with overall VE being 96.4% (95% CI: 95.9-96.8), but similar trends were observed.

Conclusion

Full infant pertussis vaccination is highly effective up to 36 months of age. These findings, consistent across both methods, emphasize importance of high infant pertussis vaccination uptake.