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 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.”
“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.
The CDC says the three- or four-week interval is still recommended for people who are moderately or severely immunocompromised, adults age 65 and older “and others who need rapid protection due to increased concern about community transmission or risk of severe disease.” There’s no data around children younger than 11, so this group is still recommended to get the second Pfizer vaccine three weeks after the first dose.
At a meeting of CDC’s independent Advisory Committee on Immunization Practices earlier this month, agency officials suggested guidance could be updated to recommend lengthening the interval between first and second doses of the mRNA vaccines.
CDC’s Dr. Sara Oliver, an epidemic intelligence service officer with the division of viral diseases, told the committee that rates of myocarditis were lower with extended intervals between first and second doses. Still, the benefits of receiving the Pfizer or Moderna vaccine are clear, regardless of time in between shots, she said.
“The benefits for both mRNA vaccines far outweigh the risk of myocarditis compared to no vaccine,” Oliver said.