Sweden's Public Health Agency on Wednesday recommended a temporary halt to the use of the Moderna Covid-19 vaccine among young adults, citing concerns over rare side effects to the heart.

It said the pause should initially be in force until December 1, explaining that it had received evidence of an increased risk of side effects such as inflammation of the heart muscle (myocarditis) and inflammation of the pericardium (pericarditis).

"The Public Health Agency has decided to pause the use of Moderna's vaccine Spikevax, for everyone born 1991 and after, for cautionary reasons," the agency said in a statement, adding that those groups should instead receive the Pfizer/BioNTech vaccine.

According to the agency, the risk seemed especially tied to the second dose of the Moderna vaccine and was more prevalent among young men and boys, and in the weeks just following the second jab.

The symptoms usually pass by themselves, but should be evaluated by a doctor, it added.

"Those who are vaccinated recently, with their first or second dose of Moderna's vaccine, don't need to feel worried because the risk is very minor, but it is good to know which symptoms you should be on the lookout for," state epidemiologist Anders Tegnell said in a statement.

Some 81,000 people in the age group have already received a first jab in Sweden, but they will not be offered a second, the agency said, adding that "discussions were ongoing about the best solution for this group."

The European Medicines Agency (EMA) authorised the emergency use of Moderna's Covid-19 vaccine for teens in July.