Sweden has suspended the use of the Moderna Covid-19 vaccine for under 30s following reports of rare side effects, while Denmark has said it will no longer offer the shot to under 18s.
The national health agency of Sweden said data pointed to an increase of myocarditis and pericarditis among youths and young adults that have received the vaccine.
Myocarditis is an inflammation of the heart which can reduce the muscle’s ability to pump blood around the body. The condition can cause rapid or abnormal heart rhythms.
Pericarditis refers to an inflammation of the pericardium, a sac-like structure with two thin layers of tissue that surround the heart to hold it in place and help it work. Symptoms include chest pain similar to that experienced by a person during a heart attack.
Sweden's public health agency said it had paused the vaccine because of “signals of an increased risk of side effects such as inflammation of the heart muscle or the pericardium”.
The connection, it said, was especially clear when it came to Moderna's vaccine Spikevax, especially after the second dose. But the agency stressed that the risk of developing such side effects was "very small".
The agency said it was now recommending the Pfizer vaccine for Swedes born in 1991 or later. About 81,000 people in that age category who have received one shot of Moderna would be offered a different coronavirus vaccine for their second dose.
Anders Tegnell, Sweden’s chief epidemiologist, said health officials 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.
Earlier this week the Swedish health agency said people aged between 12 and 15 would only get the Pfizer/BioNTech vaccine.
Denmark’s health agency also blamed concerns over myocarditis for its decision to suspend the use of the Moderna vaccine for people under 18 years.
In April it became the first European country to cease using the AstraZeneca vaccine over concerns about rare cases of blood clots.
In July, the European Medicines Agency recommended authorising Moderna’s Covid vaccine for children aged between 12 and 17, the first time the shot had been authorised for people aged under 18.
The decision came six months after the EU regulator gave the green light for the Moderna vaccine to be used on people over 18 across the 27-nation bloc.
To date, only the Pfizer vaccine is approved for children under 18 in Europe and North America.
But medicines regulators in the US and Europe have cautioned that both the Moderna and Pfizer vaccines appear linked to a rare reaction in teenagers and young adults – chest pain and heart inflammation.
The Swedish health authorities said the heart symptoms “usually go away on their own”, but they must be assessed by a doctor. It said the decision to suspend the Moderna vaccine was valid until December 1.
The conditions are most common among young men, in connection with, for example, viral infections such as Covid-19.
In 2019, about 300 people under the age of 30 were treated in hospital with myocarditis.