WASHINGTON — The US drug regulator on Friday added a warning to the literature that accompanies Pfizer Inc /BioNTech and Moderna COVID vaccine shots to indicate the rare risk of heart inflammation after its use.

Health officials said the benefits of receiving the vaccine still outweigh any risk. There have been just 12.6 heart inflammation cases per million doses for both vaccines combined.

The US Food and Drug Administration (FDA) added a warning to patient and provider fact sheets for the Pfizer and Moderna COVID-19 vaccines to indicate a rare risk of heart inflammation.

For each vaccine, the fact sheets for healthcare providers have been revised to include a warning that reports of adverse events suggest increased risks of myocarditis and pericarditis, particularly after the second dose and with onset of symptoms within a few days after vaccination, the FDA said.

As of June 11, more than 1,200 cases of myocarditis or pericarditis have been reported to the US Vaccine Adverse Event Reporting System (VAERS), out of about 300 million mRNA vaccine doses administered.

Myocarditis is the inflammation of the heart muscle and pericarditis is the inflammation of the tissue surrounding the heart. Health officials said the benefits of receiving the vaccine still outweigh any risk.

“The risk of myocarditis and pericarditis appears to be very low given the number of vaccine doses that have been administered,” Janet Woodcock, acting FDA commissioner, said in a statement to CNBC.

“The benefits of COVID-19 vaccination continue to outweigh the risks, given the risk of COVID-19 diseases and related, potentially severe, complications,” she said.

The cases appear to be notably higher in males and in the week after the second vaccine dose. The CDC identified 309 hospitalizations from heart inflammation in persons under the age of 30, of which 295 have been discharged.

Health regulators in several countries have been investigating cases of myocarditis and pericarditis, more frequently found in young men, after a shot of Pfizer or Moderna, vaccines that are based on the mRNA technology.

The latest update from FDA follows an extensive review of information and the discussion by CDC’s Advisory Committee on Immunization Practices meeting recently. — Agencies