High vaccine effectiveness against severe Covid-19 in the elderly in Finland before and after the emergence of Omicron

Abstract BackgroundThe elderly are highly vulnerable to severe Covid-19. Waning immunity and emergence of Omicron have caused concerns about reduced effectiveness of Covid-19 vaccines. The objective was to estimate vaccine effectiveness (VE) against severe Covid-19 among the elderly. MethodsThis nationwide, register-based cohort study included all residents aged 70 years and over in Finland. The follow-up started on December 27, 2020, and ended on February 19, 2022. The study outcomes were Covid-19-related hospitalization and intensive care unit (ICU) admission timely associated with SARS-CoV-2 infection. VE was estimated as 1 minus the hazard ratio comparing the vaccinated and unvaccinated and taking into account time since vaccination. Omicron-specific VE was evaluated as the effectiveness observed since January 01, 2022. ResultsThe cohort included 897932 individuals. Comirnaty (BioNTech/Pfizer) VE against Covid-19-related hospitalization was 93% (95% confidence interval [CI], 90%–95%) and 87% (84%–89%) 14–90 and 91–180 days after the second dose; VE increased to 96% (95%–97%) 14–60 days after the third dose. VE of other homologous and heterologous 3-dose series was similar. Protection against severe Covid-19 requiring ICU treatment was even better. Since January 01, 2022, Comirnaty VE was 91% (95% CI, 79%–96%) and 76% (56%–86%) 14–90 and 91–180 days after the second and 95% (94%–97%) 14–60 days after the third dose. ConclusionsVE against severe Covid-19 is high among the elderly. It waned slightly after 2 doses, but a third restored the protection. VE against severe Covid-19 remained high even after the emergence of Omicron.