Safety of bivalent omicron-containing mRNA-booster vaccines: a nationwide cohort study

ABSTRACT Background Safety data to support bivalent omicron-containing mRNA-booster vaccination are lacking. Methods In a Danish nationwide cohort study from 1 January 2021 to 10 December 2022, we examined the association between bivalent omicron-containing mRNA-booster vaccination as a fourth Covid-19 vaccine dose and risk of adverse events in individuals aged ≥50 years. Using incidence rate ratios estimated with Poisson regression, we compared the rates of hospital visits for 27 different adverse events in a 28-day main risk period following vaccination with a bivalent omicron-containing mRNA-booster vaccine as a fourth dose to reference period rates from day 29 after the third or fourth vaccine dose and onward. Secondary analyses included stratifying by sex, age, and vaccine type and assessing the associations using self-controlled case series and observed vs. expected cohort analyses. Results 1,740,417 individuals (mean age 67.8 years, standard deviation 10.7) received a bivalent omicron-containing mRNA-booster vaccine as a fourth dose. Fourth dose vaccination with a bivalent omicron-containing booster did not statistically significantly increase the rate of any of the 27 adverse outcomes within 28 days, nor when analyzed according to age, sex, vaccine type, or using alternative analytical approaches. However, post-hoc analysis detected signals for myocarditis (statistically significantly so in females), although the outcome was very rare and findings were based on few cases. No risk of cerebrovascular infarction was found. Conclusions Bivalent omicron-containing mRNA-booster vaccination as a fourth dose was not associated with an increased risk of 27 different adverse events in 50+-year-olds.