Comparative effectiveness of monovalent XBB.1.5 containing covid-19 mRNA vaccines in Denmark, Finland, and Sweden:...

To estimate the effectiveness of vaccination with a monovalent covid-19 mRNA vaccine containing the omicron XBB.1.5 subvariant against severe covid-19 disease in Denmark, Finland, and Sweden.

Target trial emulation based on registry data.

Denmark, Finland, and Sweden, 1 October 2023 to 21 April 2024.

Source population of 3 898 264 individuals eligible for vaccination with the XBB.1.5 containing covid-19 mRNA vaccine at the start of the study on 1 October 2023. Study cohort comprised 1 876 282 recipients of an XBB.1.5 containing vaccine during the study period matched with 1 876 282 non-recipients. Individuals were aged ≥65 years (mean age 75.4 years, standard deviation 7.4 years) and had received at least four doses of a previous covid-19 vaccine.

Cumulative incidences of hospital admissions and deaths related to covid-19 in a follow-up period of 24 weeks after immunisation (defined as one week after vaccination) in 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.

The associated comparative vaccine effectiveness was 57.9% (95% confidence interval (CI) 49.9% to 65.8%) against hospital admission for covid-19 (1085 v 2635 events) and 75.2% (70.6% to 79.9%) against deaths related to covid-19 disease (348 v 1458 events) after 24 weeks of follow-up. This result corresponded to 154.7 (95% CI 78.3 to 231.0) hospital admissions for covid-19 and 120.3 (110.5 to 130.2) deaths prevented per 100 000 individuals who were vaccinated with an XBB.1.5 containing vaccine. The associated comparative vaccine effectiveness was similar irrespective of sex, age group (65-74 v ≥75 years), number of doses of previous covid-19 vaccines, subgroup of co-administered seasonal influenza vaccines, and period of when either the omicron XBB or BA.2.86 sublineage was predominant. Although the observed reduction in risk was highest during the first weeks after vaccination, comparative vaccine effectiveness was well maintained after 24 weeks of follow-up.

In this study, in adults aged ≥65 years, vaccination with a monovalent XBB.1.5 containing covid-19 mRNA vaccine was associated with reduced rates of hospital admissions for covid-19 and deaths related to covid-19, during the autumn and winter of 2023-24 in Denmark, Finland, and Sweden.