Comparative effectiveness of bivalent BA.4–5 or BA.1 mRNA booster vaccines among immunocompromised individuals across three Nordic countries: A nationwide cohort study

Objectives: To estimate the effectiveness and waning of the bivalent BA.4–5 or BA.1 mRNA booster vaccine against Covid-19-related hospitalization and death in immunocompromised individuals. Methods: Nationwide analyses across Nordic countries from 1 September 2022 to 31 October 2023 using a matched cohort design. Individuals boosted with a BA.4–5 or BA.1 vaccine were matched 1:1 with unboosted individuals. The outcomes of interest were country-combined vaccine effectiveness (VE) estimates against Covid-19-related hospitalization and death at day 270 of follow-up. Waning was assessed in 45-day intervals. Results: A total of 352,762 BA.4–5 and 191,070 BA.1 booster vaccine doses were included. At day 270, the comparative VE against Covid-19-related hospitalization was 34.2% (95% CI, 7.1% to 61.3%) for the bivalent BA.4–5 vaccine and 42.6% (95% CI, 31.3% to 53.9%) for the BA.1 vaccine compared with matched unboosted. The comparative VE against Covid-19-related death was 53.9% (95% CI, 38.6% to 69.3%) for the bivalent BA.4–5 vaccine and 57.9% (95% CI, 48.5% to 67.4%) for the BA.1 vaccine. Conclusions: In immunocompromised individuals, vaccination with bivalent BA.4–5 or BA.1 booster lowered the risk of Covid-19-related hospitalization and death over a follow-up period of 9 months. The effectiveness was highest during the first months since vaccination with subsequent gradual waning.