Finland has paused the use of Moderna’s Covid-19 vaccine for men under the age of 30 one day after Sweden and Denmark implemented similar measures.
Health authorities in Helsinki said the decision was taken following reports of a rare cardiovascular side effect.
On Wednesday, Sweden said it would stop offering the jab to under 30s. Denmark later followed suit with a similar ban on Moderna for all under 18s. People who have been given one dose of the vaccine would be offered an alternative for their second.
"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," Mika Salminen from Finland’s health institute said.
Myocarditis is an inflammation of the heart muscle that can result in its ability to pump blood around the body. The condition can cause rapid or abnormal heart rhythms.
Salminen said myocarditis usually heals on its own in a few days.
As a precaution, Finland will now only give the Pfizer vaccine to boys and young men.
Denmark had already been using the Pfizer vaccine as the main option for children aged 12 to 17. It said it was pausing the use of the Moderna shots for under 18s as a “precautionary principle”.
Anders Tegnell, Sweden’s chief epidemiologist, said health authorities would “follow the situation closely and act quickly to ensure that vaccinations against Covid-19 are always as safe as possible and at the same time provide effective protection” against the disease.
Following Sweden and Denmark’s announcements on Wednesday, a spokesperson for Moderna said the company was aware of the decisions.
"These are typically mild cases and individuals tend to recover within a short time following standard treatment and rest,” the spokesperson said.
“The risk of myocarditis is substantially increased for those who contract COVID-19, and vaccination is the best way to protect against this."
Shares of Moderna fell 4.9 percent, or $16.08, to $316.11, in afternoon trading on Wednesday.
Medicine regulators in the US, EU and the World Health Organisation said the benefits of the Moderna jab, as well as the Pfizer vaccine, in preventing Covid-19 continue to outweigh the risks.
In July, the European Medicines Agency, the EU’s main drug regulator, recommended authorising the Moderna vaccine for children between 12 and 17, the first time the shot had been given approval for under 18s.
Last week Canadian health officials said data suggests there are more reported cases of heart inflammation in people who had the Moderna jab compared to those who received the Pfizer vaccine.
The Moderna jab is made in Switzerland, while the Pfizer jab is made in Belgium as well as German manufacturing sites run by its partner company BioNTech.
Both vaccines are based on mRNA technology but the Pfizer shot contains 30 micrograms of vaccine per dose compared with 100 micrograms in the Moderna vaccine.
Denmark has fully vaccinated 75 percent of its population, while 63 percent of Finnish people have been fully inoculated. Sweden has given 64 per cent of its population two doses of a coronavirus vaccine.