Previous guidelines said the second dose should be given three weeks after the first dose of the Pfizer vaccine or four weeks after the first dose of the Moderna vaccine.
Vaccines remain safe and effective within their original ranges, the CDC said, but extending the range may reduce the risk of myocarditis, a type of heart inflammation, in some populations. Rare cases of myocarditis have mostly been reported after the second dose of Covid-19 mRNA vaccines, and men aged 12 to 29 are most at risk.
“Although the absolute risk remains low, the relative risk of myocarditis is higher in men aged 12 to 39 years, and this risk can be reduced by extending the interval between the first and second dose,” said the CDC, noting that some studies in people over the age of 12 have shown that “the low risk of myocarditis associated with COVID-19 mRNA vaccines may be reduced and peak antibody responses and vaccine efficacy may be increased with an interval greater than 4 weeks”.
“An 8-week break may be ideal for some people aged 12 and over, especially men aged 12 to 39,” the new guide says.
The CDC says the three- or four-week break is still recommended for people who are moderately or severely immunocompromised, adults 65 and older, “and others who need prompt protection due to heightened concern about community transmission or risk of serious illness”. . There are no data for children under 11, so this group is still recommended to receive the second Pfizer vaccine three weeks after the first dose.
Booster doses continue to be recommended for most people five months after a two-dose primary series of an mRNA vaccine or two months after a Johnson & Johnson single-dose primary series.
At a meeting of the CDC’s Independent Advisory Committee on Immunization Practices this month, agency officials suggested the guidelines could be updated to recommend extending the interval between first and second doses. mRNA vaccines.
CDC’s Dr. Sara Oliver, head of epidemic intelligence in the Division of Viral Diseases, told the committee that myocarditis rates were lower with extended intervals between the first and second dose. Still, the benefits of getting the Pfizer or Moderna vaccine are clear, regardless of the time between injections, she said.
“The benefits of both mRNA vaccines far outweigh the risk of myocarditis compared to either vaccine,” Oliver said.
Lessons learned from longer vaccination schedules
Canada opted to delay a second dose for up to sixteen weeks to vaccinate more people when shots were scarce, then adjusted to an eight-week interval.
Several studies have shown that the delay reduces the already rare risk of myocarditis or pericarditis after vaccination, and there is an added benefit.
“With the subsequent dosing of the second dose, we actually got a better immune response, and it was better in quantity and quality,” Duerr said.
Studies have also shown that a delay in getting the second dose will not leave people unprotected between injections.
With the longer dosing regimen, antibody levels fell slightly between the first and second doses, but T cells – which promote antibody protection and can kill infected cells – were well maintained between doses.
After the second dose of the longer regimen, antibody levels exceeded those generated with the shorter regimen.
“So we were able to be sure that people were protected during that longer interval, and what we found was that actually after the second dose in that longer interval, after the second vaccine, what we saw is that the antibodies fared much better, especially when we tested it against a few variants, including Delta,” Payne said.
Additionally, after the delayed dose, a greater proportion of T cells were helper T cells, which are essential for long-term immune memory with the longer dosing regimen, according to their study.
“It implies that they get a slightly better immune response,” Payne said.