The Moderna Covid jab carries a slight risk of usually non-serious heart problems a study of the entire population of Denmark found Friday.
Incidences of myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the tissue surrounding the heart) after mRNA jabs from Pfizer and Moderna had been noted in vaccine safety reports and small-scale studies.
"Vaccination with mRNA-1273 (Moderna) was associated with a significantly increased rate of myocarditis or myopericarditis, especially among individuals aged 12-39 years," the study said.
It said that vaccination with Pfizer was only associated with an increased risk among women.
The study published in the BMJ medical journal is the first to look at these side effects in an entire population.
While it confirms the risks, it insists they are slight and that the risks posed by Covid infection are greater.
It noted that among the vaccinated who developed myocarditis or pericarditis "only a few" had severe outcomes.
"Our finding of a low absolute risk of myocarditis or myopericarditis with BNT162b2 or mRNA-1273 vaccination supports the overall benefits of such vaccination on an individual, societal, and global level," the study concluded.