Previous guidance said the second dose 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 initial intervals, the CDC said, but prolonging the interval can 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 men aged 12 to 29 are at greatest risk.
“Although the absolute risk remains small, the relative risk of myocarditis is higher for men aged 12-39 years, and this risk can be reduced by prolonging the interval between the first and second dose,” the CDC said, noting some studies in older people . than 12 have shown “the small risk of myocarditis associated with mRNA COVID-19 vaccines can be reduced and maximal antibody responses and vaccine efficacy can be increased at an interval of more than 4 weeks.”
“An 8-week interval may be optimal for some people aged 12 years and older, especially for men aged 12-39 years,” says the new guide.
The CDC says the three- or four-week interval is still recommended for people who are moderately or severely immunocompromised, adults aged 65 and older “and others who need prompt protection due to increased concern about societal transmission or risk of serious illness. . ” There are no data on children under 11 years of age, so this group is still recommended to receive the second Pfizer vaccine three weeks after the first dose.
At a meeting of the CDC’s independent advisory committee on immunization practices earlier this month, agency officials suggested that the guidelines could be updated to recommend extending the range between the first and second doses of mRNA vaccines.
CDC’s Dr. Sara Oliver, an epidemic intelligence officer with the division of viral diseases, told the committee that the incidence of myocarditis was lower with longer intervals between the first and second dose. Still, the benefits of receiving the Pfizer or Moderna vaccine are clear, regardless of the time between shots, she said.
“The benefits of both mRNA vaccines far outweigh the risk of myocarditis compared to no vaccine,” Oliver said.