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

Vaccine effectiveness (vaccine effectiveness) 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.

Among 225 cases, 88% were diagnosed serologically and 12% by culture or PCR.

Vaccine effectiveness remained stable up to 8 years post-vaccination without clear waning.

The 2aP preschool booster seems to provide moderate and sustained protection against pertussis compared to children without the booster.