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 896,220 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.Peer reviewe