The US Centers for Disease Control and Prevention (CDC) said that more than 1,200 cases of myocarditis and pericarditis have been reported across the country in young people who received the mRNA Covid-19 vaccines.
Experts of CDC's Advisory Committee on Immunization Practices met on Wednesday to review risks of these rare heart problems in the vaccinated younger population, reports Xinhua news agency.
A total of 1,226 cases of myocarditis and pericarditis were reported in young people who received the Pfizer or Moderna vaccines through June 11, according to the CDC.
There have been 267 cases reported after receiving one dose of the mRNA vaccines, and 827 reported cases after two doses.
There are 132 additional cases where the number of doses received is unknown, according to the CDC.
Of these cases, 309 developed symptoms and were hospitalized, and 295 people have been discharged. About 79 per cent of them have fully recovered.
Nine people were still hospitalized, with two in intensive care as of June 11, according to the CDC.
Most cases have been mild, with symptoms like fatigue, chest pain and disturbances in heart rhythm that quickly clear up within a day or so.
Males under 30 make up the majority of the cases. More cases occurred after the second dose than the first, according to the CDC.
Experts said they will need to follow up with patients in the months ahead in order to get a complete picture of the impact.
Clinical presentation of myocarditis cases following vaccination has been distinct, occurring most often within one week after the second dose, with chest pain as the most common presentation, said Grace Lee, who chairs the committee's safety group.
CDC officials are gathering more data to fully understand the potential risks, how to manage it and whether there are any long-term issues, she said.
Experts stressed this occurrence is rare, about 12.6 heart inflammation cases per million doses.
Officials emphasized that the benefits of the vaccines outweigh the risks, and noted that for every million doses of mRNA vaccine given, there are far more Covid-19 cases and hospitalizations prevented compared to the number of potential myocarditis cases.
About 45.4 per cent of the US population is fully vaccinated, and 53.6 per cent of the population has received at least one shot, according to CDC data.
Roughly 144.8 million of people 18 and older in the US are fully vaccinated.