Introduction of pertussis vaccination for pregnant women during the 2024 outbreak in Finland

Abstract Issue Acellular pertussis containing vaccination (aP) during pregnancy has been shown to be safe and effective in reducing burden of pertussis in infants. Until 2024, Finland was one of the few countries in Europe not recommending aP booster during pregnancy. Description of the problem In 2024, a record high number of 2814 pertussis cases were reported to the National Infectious Diseases Register (NIDR), maintained by the Finnish Institute for Health and Welfare (THL). The yearly mean in 2010-2019 was 338 cases (range: 165-557). As a response to the outbreak and to protect infants at risk of severe infection, on 23.8.2024 THL recommended a prenatal booster aP. This recommendation was originally temporary until the end of January 2025, and has since been extended. Our aim was to assess the coverage of prenatal aP and the recommendation impact, to support decision-making for further policy. Vaccination coverage (VC) among pregnant women was calculated as a percentage of persons vaccinated against pertussis among all those who had a coded maternity clinic visit. Pertussis cases were identified from NIDR, and caregivers of infants younger than three months were contacted to enquire about prenatal aP uptake. Results In April 2025, VC was 57%. 28 pertussis cases in infants younger than three months were reported to NIDR between 1.1.2024 and 10.7.2025. Among them, 6 infants (21%) were born at least two weeks after the recommendation was issued. None of the mothers had received the aP booster in a timely manner. Lessons VC rapidly rose following the recommendation and a national vaccine effectiveness study is planned. International evidence shows that pertussis outbreaks have become more frequent after switching from whole-cell vaccines to aP, and studies also confirm the safety and effectiveness of prenatal aP. Therefore, following THL's advice, the Ministry of Health decided to permanently include prenatal aP in the national immunisation program as of 1.7.2025. Key messages • Since the initiation of pertussis vaccination in pregnant women in Finland, no infant whose mother was vaccinated in a timely manner during pregnancy has had laboratory-confirmed pertussis. • In countries where pertussis vaccination for pregnant women is not yet recommended, a good coverage can be rapidly reached in case of an outbreak.