Finland's institute for health and welfare has said it will pause the use of Moderna's Covid-19 vaccine for men born in 1991 and later due to reports of a rare cardiovascular side effect, Reuters reports.

"A Nordic study involving Finland, Sweden, Norway and Denmark found that men under the age of 30 who received Moderna Spikevax had a slightly higher risk than others of developing myocarditis," director Mika Salminen from the health institute said.

Salminen said myocarditis, which means heart muscle inflammation, usually heals on its own in a few days. He said as a precaution Finland would only give Pfizer's vaccine to boys and young men.

A Moderna spokesperson had said late on Wednesday that the company was aware of the decisions by the Swedish and Danish regulators.