"Main text
... The aim of this study was to estimate the effectiveness of BA.1 and BA.4-5 bivalent COVID-19 vaccines against severe COVID-19 outcomes up to five months after vaccination in Finland based on national register data. Since bivalent boosters were primarily offered to individuals who had received at least two monovalent doses, our study was restricted to these vaccinees...
Results
Among the elderly, we observed altogether 2,013 hospitalisations due to COVID-19, 1,167 deaths due to COVID-19 and 984 deaths in which COVID-19 was a contributing factor. In September–December 2022, during the first 14–30 and 31–60 days since vaccination, a bivalent booster lowered the risk of hospitalisation and death... Thereafter, in January–March 2023, the HRs [hazard ratios] were higher... Overall, in January–March 2023, the risk of severe COVID-19 outcomes was similar between those who did and those who did not receive a bivalent booster at any time since vaccination...
Among the chronically ill we observed altogether 278 hospitalisations due to COVID-19, 18 deaths due to COVID-19 and 20 deaths in which COVID-19 was a contributing factor. In September–December 2022, the HR of hospitalisation due to COVID-19 was 1.01 (95% CI 0.43–2.40) for days 14–30 since bivalent vaccination and 1.81 (0.87–3.76) for the subsequent 30 days. In January–March 2023, that HR ranged between 0.74 (95% CI 0.16–3.30) and 3.40 (1.34–8.66) during days 31–150."
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