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.
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.
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.
“While absolute risk remains small, the relative risk for myocarditis is higher for males ages 12-39 years, and this risk might be reduced by extending the interval between the first and second dose,” the CDC said, noting that some studies in people older than 12 have shown “the small risk of myocarditis associated with mRNA COVID-19 vaccines might be reduced and peak antibody responses and vaccine effectiveness may be increased with an interval longer than 4 weeks.”
Read more at CNN.