June 23 (Reuters) – The U.S. Food and Drug Administration said on Wednesday it plans to move quickly to add a warning about rare cases of heart inflammation in adolescents and young adults to fact sheets for the Pfizer/BioNTech (PFE.N), and Moderna (MRNA.O) COVID-19 vaccines.

U.S. Centers for Disease Control and Prevention (CDC) advisory groups, meeting to discuss reported cases of the heart condition after vaccination, found the inflammation in adolescents and young adults is likely linked to the vaccines, but that the benefits of the shots appeared to clearly outweigh the risk.

Moderna shares closed down 4.2%, while Pfizer fell 1.4%.

Health regulators in several countries have been investigating whether the Pfizer/BioNTech and Moderna shots using new mRNA technology present a risk and, if so, how serious.

The CDC said that patients with heart inflammation following vaccination generally recover from the symptoms and do well.

The U.S. Department of Health And Human Services, joined by leading U.S. doctors groups and public health officials, put out a statement underscoring that the vaccines are safe and effective and that the heart side effect is “extremely rare.”

“We strongly encourage everyone age 12 and older who are eligible to receive the vaccine under Emergency Use Authorization to get vaccinated,” it said.

Doctors and hospitals have been warned by the CDC to watch for symptoms of myocarditis or pericarditis, and the FDA warning will further raise awareness.

The number of Americans receiving their first dose of COVID-19 vaccine is down about 85% since peaking in mid-April, and will likely fail to meet President Joe Biden’s goal to have delivered at least one shot to 70% of adults by July 4.