In 2 studies of adults aged 50 years or older, bivalent mRNA booster vaccines with the original SARS-CoV-2 and omicron BA.4-5 or BA.1 subvariants were effective and safe as a fourth COVID-19 vaccine dose, researchers report. In a comparative effectiveness study, the fourth dose with the bivalent product reduced rates of COVID-19–related hospital admissions and deaths compared with 3 doses. No increased risk of 27 different adverse events was found in the safety analysis.
Conducted in Denmark, Finland, Norway, and Sweden from July 1, 2022, to April 10, 2023, the comparative effectiveness study investigators concluded: “The protection afforded by the bivalent BA.4-5 and BA.1 boosters [Comirnaty (Pfizer-BioNTech) or Spikevax (Moderna)] did not differ significantly when directly compared, and any potential difference would most likely be very small in absolute numbers.”
Among 1.7 million adults aged 50 years or older in Denmark who received a BA.4-5 or BA.1 bivalent mRNA booster vaccine as a fourth dose, the incidence rate for ischemic cardiac events and other adverse events within 28 days of vaccination was not increased. “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,” the investigators write. “No risk of cerebrovascular infarction was found (incidence rate ratio 0.95, 95% confidence interval 0.87 to 1.05; 644 v 9687 events).”