OBJECTIVE: To examine the association between the omicron adapted bivalent mRNA covid-19 booster vaccines received as a fourth dose and risk of adverse events. DESIGN: Nationwide cohort study.Denmark. PARTICIPANTS: 2 225 567 adults aged ≥50 years who received three covid-19 vaccine doses during the study period, 1 January 2021 to 10 December 2022. MAIN OUTCOME MEASURES: The main outcome measure was rates of hospital visits for 27 different adverse events in a 28 day main risk period after vaccination with a bivalent omicron adapted mRNA booster vaccine as a fourth dose compared with reference period rates from day 29 after the third or fourth vaccine dose and onward. RESULTS: 1 740 417 adults (mean age 67.8 years, standard deviation 10.7 years) received a bivalent mRNA vaccine as a fourth dose. Fourth dose vaccination with a bivalent mRNA vaccine was not associated with a statistically significant increased rate of any of the 27 adverse outcomes within 28 days (eg, incidence rate ratio of 0.95, 95% confidence interval of 0.87 to 1.04 for ischaemic cardiac events based on 672 v 9992 events during the compared 28 day risk and reference period, respectively), nor when analysed according to age, sex, or vaccine type, or when using alternative analytical approaches. However, post hoc analysis detected signals for myocarditis (statistically significant in female participants), although the outcome was rare and findings were based on few cases. No risk of cerebrovascular infarction was found (incidence rate ratio 0.95, 95% confidence interval 0.87 to 1.05; 644 v 9687 events). CONCLUSIONS: The use of bivalent mRNA vaccines as a fourth vaccine dose against covid-19 was not associated with an increased risk of 27 different adverse events in adults aged ≥50 years.