Effectiveness and impact of the 2-component acellular pertussis vaccine as a preschool booster in Finland - A register-based study.

In 2005, Finland replaced the whole-cell pertussis (wP) 3 + 1 schedule with an acellular pertussis (aP) 2 + 1 infant schedule supplemented by a DTaP-IPV preschool booster at age 4 in the National Immunization Program (NIP). Recent pertussis resurgence and uncertainty about the duration of vaccine-induced protection have raised concerns. We assessed the effectiveness, duration of protection and population-level impact of the preschool booster. We conducted a nationwide population-based register study in Finland (1995-2019). Vaccine effectiveness (VE) of a two-component (2aP) preschool booster against laboratory-confirmed pertussis was estimated by comparing children with and without the booster, followed from age of 4.25 years during 2011-2019 using a covariate-adjusted Cox proportional hazards model. Population-level impact was assessed by comparing pertussis incidence before (1995-2002; wP primary series without preschool booster) and after (2012-2019; aP primary series with the booster) its introduction. The primary cohort included 466 240 children (94% vaccination coverage). Among 225 cases, 88% were diagnosed serologically and 12% by culture or PCR. VE against any laboratory-confirmed pertussis during 5-year follow-up was 51% (95% CI, 27-67), with VE of 86% (95% CI, 67-94) against PCR/culture-confirmed and 37% (95% CI, -1-60) against serology-confirmed cases. VE remained stable up to 8 years post-vaccination without clear waning. Following booster introduction in the NIP, pertussis incidence decreased by 65% (95% CI, 60-69) among children ≥4.25 years and by 39% (95% CI, 30-48) among those ≤3.75 years, indicating both direct and indirect effects. The 2aP preschool booster seems to provide moderate and sustained protection against pertussis compared to children without the booster. Effectiveness was notably higher against PCR/culture-confirmed cases throughout the follow-up, likely due to lower specificity of serology and differences in diagnostic practices by disease manifestations. These findings support continued use of the preschool booster in Finland.