A Centers for Disease Control and Prevention advisory panel has found a “likely association” between a rare heart inflammation and mRNA Covid-19 vaccines in adolescents, although it said that the benefits of the jabs far outweigh their risks.

The health agency is monitoring cases of myocarditis and pericarditis that have been reported after first and second doses of the Moderna and BioNTech/Pfizer mRNA vaccines in people aged over 12.

Myocarditis is inflammation of the heart muscle, and pericarditis is inflammation of the membrane that surrounds the heart.

There were 323 confirmed cases among people who received the vaccine of either or both conditions in the US up to June 11, with 309 of people needing hospital treatment. Nearly 80 per cent of the people with confirmed cases have fully recovered, the CDC’s Vaccine Safety Technical Work Group said.

The rare side-effect has been primarily seen in young people and is most common in men aged 30 years and younger.

In people aged 12-39 there have been 4.4 reports per million vaccine doses administered of either issue. The report rate rose to 12.6 per million after the second dose, experts said at an advisory meeting on Wednesday.

The majority of patients experienced symptoms, which include chest pain, shortness of breath and heart palpitations, within a week of vaccination.

More than 150m Americans are fully vaccinated, according to CDC figures.

The heart-related issues are the second type of unusual side-effects detected as Covid-19 vaccinations have been distributed rapidly around the world. The AstraZeneca and Johnson & Johnson vaccines were linked to rare blood clots, and the rollout of both jabs was paused in many countries earlier this year while scientists investigated the cause.

CDC panel members recommended deferring the second dose of an mRNA vaccine — a category which includes jabs made by BioNTech/Pfizer and Moderna — if an adolescent experiences the rare condition after the first shot.

“This is an extremely rare side-effect, and only an exceedingly small number of people will experience it after vaccination,” doctors from the US health department, CDC and others said in a statement following the meeting.

“Most cases are mild, and individuals recover often on their own or with minimal treatment. In addition, we know that myocarditis and pericarditis are much more common if you get Covid-19, and the risks to the heart from Covid-19 infection can be more severe,” they added.

Doran Fink, deputy director at the Food and Drug Administration’s office of vaccine research, said the regulator would “move rapidly” to include warning statements for healthcare providers and vaccine recipients about the reaction.

Moderna shares closed 4.2 per cent lower on Wednesday, while BioNTech’s US-listed shares declined 3 per cent and Pfizer lost 1.4 per cent.