Bivalent booster effectiveness against severe COVID-19 outcomes in Finland, September 2022 — January 2023

AbstractBivalent COVID-19 vaccines were introduced in 2022 but knowledge of how their effectiveness against severe COVID-19 outcomes is sustained over time is currently limited. In Finnish register-based cohort analyses, we compared the risk of severe COVID-19 outcomes among those who received bivalent vaccination (exposed) between 1 September 2022 and 31 January 2023 to those who did not (unexposed). Among elderly aged 65–120 years, bivalent vaccination reduced the risk of hospitalisation and death due to COVID-19. Among the elderly the hazard ratios comparing exposed and unexposed ranged from 0.36 to 0.43 during the first 14–30 days since bivalent vaccination but signs of waning were observed as soon as two months after vaccination. Among the chronically ill aged 18–64 years bivalent vaccination did not reduce the risk of severe COVID-19 outcomes. These results are crucial for further developing COVID-19 vaccination programme worldwide.