The U.S. Centers for Disease Control and Prevention said on Thursday it had received reports of eight cases of myocarditis, a type of heart inflammation, in children aged 5-11 years who received Pfizer and BioNTech's COVID-19 vaccine.

The CDC had previously said that reporting rates of myocarditis for boys aged 16 to 17 could be more than 69 cases per million second doses administered and around 40 cases per million second doses in boys aged 12-15 years old.

The CDC did not say whether it believes there is a link between the myocarditis cases and the vaccine, or disclose the rate of myocarditis in the age group without vaccination.

The agency said there had been over 7 million vaccine doses in the 5-11 age group at the time it examined the data, with 5.1 million first doses and 2 million second doses. The cases had a mild clinical course, the CDC said.

The cases were reported in the U.S. Vaccine Adverse Event Reporting System and presented by the CDC to a panel of its expert advisers.