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

Background The 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. Methods This nationwide, register-based cohort analysis included all residents aged 70 years and over in Finland. The follow-up started on December 27, 2020, and ended on March 31, 2022. The outcomes of interest were Covid-19-related hospitalization and intensive care unit (ICU) admission timely associated with SARS-CoV-2 infection. VE was estimated as one 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 1, 2022. Results The cohort included 896220 individuals. Comirnaty (BioNTech/Pfizer) VE against Covid-19-related hospitalization was 93% (95% CI 89%–95%) and 85% (95% CI 82%–87%) 14–90 and 91–180 days after the second dose; VE increased to 95% (95% CI 94%–96%) 14–60 days after the third dose. VE of other homologous and heterologous three dose series was similar. Protection against severe Covid-19 requiring ICU treatment was even better. Since January 1, 2022, Comirnaty VE was 98% (95% CI 92%–99%) and 92% (95% CI 87%–95%) 14–90 and 91–180 days after the second and 98% (95% CI 95%–99%) 14–60 days after the third dose. Conclusions VE against severe Covid-19 is high among the elderly. It waned slightly after two doses, but a third restored the protection. VE against severe Covid-19 remained high even after the emergence of Omicron.