Bivalent booster effectiveness against severe COVID-19 outcomes in Finland, September 2022 – March 2023

Bivalent COVID-19 vaccines were introduced in 2022 but knowledge of their effectiveness against severe COVID-19 outcomes 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 September 2022 and March 2023 to those who did not (unexposed). Among elderly aged 65–110 years, bivalent vaccination reduced the risk of hospitalisation and death due to COVID-19 in September–December 2022; the hazard ratios comparing exposed and unexposed ranged from 0.37 to 0.45 during the first 31–60 days since bivalent vaccination. However, in January–March 2023 the effect disappeared possibly indicating immune evasion of new SARS-CoV-2 variants, waning of vaccine effectiveness and increased presence of hybrid immunity. Among the chronically ill aged 18–64 years bivalent vaccination did not reduce the risk of severe COVID-19 outcomes. These results are important for developing COVID-19 vaccines and programmes worldwide.