Extending the interval between the first two doses of the most widely used COVID-19 vaccines in the country to eight weeks for young men can reduce the rare risk of heart inflammation, U.S. health officials said.

The side effect, which has been linked to both Pfizer and Moderna's COVID-19 vaccines, is higher in 12- to 39-year-old men, and therefore the eight-week interval could be optimal for some people aged 12 years and older, the U.S. Centers for Disease Control and Prevention said on Tuesday.

The recommended interval between the first two shots will remain three weeks for Pfizer's vaccine and four weeks for Moderna's vaccine in other population groups, the CDC said.

Myocarditis is a rare side effect seen with mRNA vaccines - the technology behind both the Pfizer/BioNTech and Moderna shots. It appears to be more common among young men.