Comparative effectiveness of the monovalent XBB.1.5-containing covid-19 mRNA vaccine across three Nordic countries

ObjectiveTo estimate the effectiveness of vaccination with a monovalent XBB.1.5-containing covid-19 mRNA vaccine against severe covid-19 across three Nordic countries. DesignNationwide cohort studies, using target trial emulation. SettingDenmark, Finland, and Sweden, from 1 October 2023 to 29 February 2024. ParticipantsIndividuals aged [≥]65 years who had previously received at least four covid-19 vaccine doses. Main outcome measures: Cumulative incidences of covid-19 hospital admission and death for 12 weeks after immunisation (defined as 1 week after vaccination) among recipients of an XBB.1.5-containing covid-19 mRNA vaccine and matched non-recipients. Cumulative incidences were used to calculate comparative vaccine effectiveness (1-risk ratio) and risk differences. ResultsDuring autumn and winter 2023-2024, a total of 1,867,448 1:1 matched pairs of XBB-containing covid-19 mRNA vaccine recipients and non-recipients were included (mean age 75.4 years, standard deviation 7.4 years). The comparative vaccine effectiveness was 60.6% (95% confidence interval, 46.1% to 75.1%) against covid-19 hospital admission (930 v 2,551 events) and 77.9% (69.2% to 86.7%) against covid-19 related death (301 v 1,326 events) at 12 weeks of follow-up. This corresponded to 191.1 (95% confidence interval, 50.2 to 332.1) covid-19 hospital admissions and 109.2 (100.2 to 118.1) deaths prevented per 100,000 individuals vaccinated with an XBB.1.5-containing vaccine. The comparative vaccine effectiveness was similar across sex, age (65-74/[≥]75 years), number of previous covid-19 vaccine doses received, and seasonal influenza vaccination co-administration subgroups and periods of either omicron XBB- or BA.2.86-sublineage dominance. While the protection was highest during the first weeks after vaccination, it was well-preserved at end of week 12 of follow-up. ConclusionAmong adults aged [≥]65 years, vaccination with a monovalent XBB.1.5-containing covid-19 mRNA vaccine reduced the rates of covid-19 related hospital admission and death during autumn and winter 2023-2024 across three Nordic countries.