People who receive Moderna’s COVID-19 vaccine have a greater risk of developing chronic hives, according to researchers in Denmark.
The Danish Medicines Agency review of data from Denmark and the European Union validated a safety signal that arose for chronic hives, or chronic urticaria, and Moderna’s shot, the agency said on March 20.
Of 360 cases reported in Europe following the Moderna or Pfizer-BioNTech vaccine, 58 were deemed probably caused by vaccination and 228 were determined to be possibly caused by the vaccination, Martin Zahle Larsen from the Danish Medicines Agency said in a statement.
Most of the cases were reported by patients, doctors, or pharmaceutical companies.
The study found that in Denmark, it was expected based on background rates of chronic hives that 175 people who received Pfizer’s shot would experience chronic hives following vaccination and that 18 people who received Moderna’s shot would experience the issue.
While the 105 reported cases after Pfizer vaccination came in under the expected number, the 55 reported cases following Moderna vaccination came in well above the expected number.
The risk of developing chronic hives was calculated to be three times higher for Moderna recipients, compared to the general population. Researchers also stratified the risk by gender and age and found the risk was the highest—5.2 times higher than the background rate—among young men.
Most cases of chronic hives occurred from 7 to 13 days following vaccination.
The results of the study are the validation of a safety signal, or a sign that a vaccine or vaccines causes a specific health issue, Danish authorities said in a document describing the results.
Mr. Larsen, though, told Danish media that additional studies are required to confirm a connection and that scientists think the cases stem from the vaccine’s impact on the immune system.
The cases began being reported after the COVID-19 vaccines were introduced and Norway reported a safety signal for chronic hives in late 2021. The Danish Medicines Agency examined reports of chronic hives after Moderna vaccination but reached the position that the cases were not strong enough to establish a safety signal, it said in September 2022. But data from the county’s compensation system for vaccine injuries subsequently indicated an investigation into the possible side effect should be reopened, prompting a fresh look that led to the new results.
The review was strengthened by gaining access to medical records from the compensation, the agency said.
Moderna did not return a request for comment.
Based in part on the results, the European Medicines Agency’s Committee for Medicinal Products for Human Use has recommended labeling for Moderna’s shot be updated to list chronic hives as a possible side effect. Non-chronic hives is already listed as a possible side effect. If the European Union Commission approves the change, the labeling will be updated to include chronic hives.
Previous Studies
Some previous studies have detailed cases of chronic hives following COVID-19 vaccination.
U.S. researchers, for instance, reported in 2022 three new chronic hives cases after Pfizer and Moderna vaccination, including one case in a 24-year-old woman who received a booster of Moderna despite suffering persistent skin problems after the first and second doses.
Swiss researchers in 2023 said they analyzed new chronic hives cases after Pfizer and Moderna vaccination and that the results suggested a link between a booster dose of Moderna’s vaccine and the health problem.
U.S. researchers in January reported a case series of seven patients who developed chronic urticaria within weeks of Moderna vaccination and said the series indicated a “potential correlation” between the shot and the issue. Two of the patients, they noted, went on to receive a Pfizer dose with no problem.
Hives have also been associated with COVID-19, but researchers aren’t sure whether there is a causal link.
