A man informs citizens of things to beware when receiving COVID-19 vaccines at a vaccination festival in New Orleans, New Orleans, Louisiana, the United States, on May 29, 2021. Photo: Xinhua

US health authorities plan to update official guidance about administering mRNA COVID-19 vaccines to adolescents and young adults after finding a likely link to rare cases of heart inflammation, but say the overall benefits still "clearly" outweigh the risks. 

The decision was announced during a meeting of experts convened by the US Centers for Disease Control and Prevention (CDC) on Wednesday that reviewed 323 confirmed instances of myocarditis and of pericarditis, inflammation of the lining surrounding the heart, among people under 30 following vaccination.

"Fortunately it's an extraordinarily rare event, and when the event happens it's generally quite mild," Henry Bernstein, a pediatrician at Cohen Children's Medical Center in New York and member of the expert panel, told AFP.

He added he would continue to strongly advise parents to vaccinate their adolescents.

Of the 323 cases, 309 were hospitalized, 295 were discharged, nine remain hospitalized with two in intensive care, and there is no outcome data for five cases.

The cases are predominantly among males, most occur after the second dose, and there are no confirmed deaths. 

The data is current as of June 11, when more than 50 million doses of both the Pfizer and Moderna mRNA vaccines had been administered to people aged 12-29 in the US.

While the case numbers are low, they are still higher than what would be expected in these age groups and researchers are studying possible biological triggers.

"Myocarditis is a rare disease but it's not a new disease, it's been around for a while," Matthew Oster, a member of the CDC's COVID-19 vaccine task force told the meeting, noting the estimated background rate of myocarditis is about 0.8 per 100,000 children per year.

Oster said the illness was thought to be caused by viruses in some cases, although there could be other triggers.

"It does appear that mRNA vaccine may be a new trigger for myocarditis," he said.

According to the CDC, for every million second doses given to 12-to-17-year-old boys, 5,700 COVID-19 cases, 215 hospitalizations, 71 ICU admissions and two deaths across 120 days would be prevented. 

Between 56 to 69 cases of myocarditis would be expected as a result of the vaccines.

"Currently, the benefits still clearly outweigh the risks for COVID-19 vaccination in adolescents and young adults," CDC scientists said in a presentation.

The CDC and Food and Drug Administration plan to update their advice and warning labels to reflect the potential risks.