The interval between first and second doses of the Pfizer and Moderna Covid-19 vaccines may be as long as eight weeks for certain people, the US Centers for Disease Control and Prevention said in vaccine guidance updated Tuesday, CNN reports.

Previous guidance said the second doses should be administered three weeks after the first shot of the Pfizer vaccine or four weeks after the first shot of the Moderna vaccine.

Reduce the risk of myocarditis

The vaccines remain safe and effective at their original intervals, the CDC said, but extending the interval may reduce the risk of myocarditis, a type of heart inflammation, in some populations. Rare cases of myocarditis have been reported primarily after the second dose of mRNA Covid-19 vaccines, and males ages 12 to 29 are at highest risk.

"An 8-week interval may be optimal for some people ages 12 years and older, especially for males ages 12--39 years," the new guidance says.

"While absolute risk remains small, the relative risk fo ...