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.