Nearly 400 children between the ages of 12 and 17 were diagnosed with heart inflammation after receiving the Pfizer-BioNTech COVID-19 vaccine, according to a study published by the CDC on July 30.

The condition, also known as myocarditis, occurred mostly in young boys, the study found. Heart inflammation was not identified as an adverse reaction during the safety trials for the vaccine.

The CDC conducted the study by reviewing reports of adverse reactions to the Vaccine Adverse Event Reporting System (VAERS) between Dec. 14, 2020, and July 16, 2021.

Of the 8.9 million children who received the vaccine as of July 16, VAERS received a total of 9,246 reports of adverse reactions, 90.7 percent of which consisted of “nonserious adverse events.” The 397 reports of heart inflammation made up 4.3 percent of the total.