US health authorities said they were looking into a small number of reports of heart inflammation among some younger people who received Covid vaccines, with no clear link established for the moment.
There were "relatively few reports of myocarditis," primarily in adolescents and young adults, following doses of mRNA vaccines like Pfizer and Moderna, the US Centers for Disease Control and Prevention (CDC) said.
"Most cases appear to be mild, and follow-up of cases is ongoing," the CDC said, adding that reports were more often from males who had gotten their second dose.
The agency did not publish detailed information such as the number of reports or the ages of those affected.
"Within CDC safety monitoring systems, rates of myocarditis reports... following Covid-19 vaccination have not differed from expected baseline rates," CDC added.
The United States started in mid-May allowing children aged 12-15 to get vaccinated against the novel coronavirus.