Informing Public Health Vaccination Policies Through Immune Surveillance and Serological Studies

Abstract Immune surveillance can be used to assess population immunity from vaccination and infections to guide vaccination strategies, including booster needs and protection gaps. The Finnish Institute for Health and Welfare (THL) has monitored immunity trends by combining serological studies with vaccination coverage data for several pathogens. These studies have shaped e.g. national COVID-19 vaccination recommendations. Immunity studies demonstrated that a prior SARS-CoV-2 infection boosted immune responses to a later vaccine dose, justifying the recommendation that one infection could be considered equivalent to one vaccine dose. Serosurveillance showed that by early 2022, widespread hybrid immunity from infection and vaccination had developed particularly in younger adults. This evidence led to limiting covid-19 booster recommendations to older adults and high-risk groups. While most previously infected individuals retained antibodies one year after infection, neutralizing antibodies against emerging variants were lower, especially after mild disease. This supported targeted booster campaigns for those at risk of severe illness. Immune responses were evaluated after booster doses in immunocompromised individuals, showing improved, but in certain patient groups suboptimal responses. Immunity was strongly enhanced by post-vaccine infections during the Omicron era among healthcare workers, suggesting little added benefit from additional boosters in healthy adults by late 2023; this evidence supported the decision not to recommend further boosters for healthcare personnel. These insights highlight how real-time serosurveillance supports tailored, evidence-based vaccination policies across different population groups. Key messages • Population immunity and immunogenicity studies support more targeted and effective vaccination strategies. • Infectious disease surveillance for public health purposes should also include immune surveillance.