Associations between mRNA COVID-19 vaccination and urticaria: a nationwide registry-based cohort study in Denmark.

Objectives: To investigate the association between mRNA COVID-19 vaccination and the risk of urticaria.

Study design: Nationwide observational cohort study.

Setting: Healthcare registers in Denmark.

Participants: The vaccinated cohort included Danish residents aged ⩾5 years who received at least one dose of Comirnaty (Pfizer-BioNTech, BNT) or Spikevax (Moderna, MOD) in December 2020 to July 2023; the pre-pandemic (2017-19) general population aged ⩾5 years formed the comparator cohort.

Methods: Urticaria-free participants were followed from the vaccination date until the earliest of the incident chronic or other types of urticaria diagnosis, COVID-19 infection, end of the 90-day outcome risk window, death, emigration, or October 2023. The expected number of urticaria events was computed using indirect standardisation of the pre-pandemic urticaria incidence rates. Standardised incidence ratios (SIRs) with corresponding 95% confidence intervals (CIs) were calculated overall and across sex, age and vaccine sequence-specific strata.

Results: We ascertained 4,700,301 vaccinated and 5,480,146 pre-pandemic comparators. Following all vaccine product sequences, SIRs (95% CI) were 0.83 (0.71-0.96) for chronic urticaria and 1.14 (95% CI 1.03-1.25) for other types of urticaria. Among MOD schedule adherers, the risk was three to fourfold increased for chronic (SIR 3.00, 95% CI 2.27-3.88) and other types of urticaria (SIR 3.65, 95% CI 3.06-4.31). Sensitivity analyses of dose specific effects confirmed a fourfold increased risk after MOD vaccine sequence but not BNT.

Conclusions: Following the most recent mRNA vaccination dose, MOD vaccine schedule adherers had a higher than expected incidence of chronic and other types of urticaria. We found no association between BNT vaccination schedule and increased risk of urticaria.