Some people receiving their first injections of the Pfizer-BioNTech and Moderna coronavirus vaccines should consider waiting longer for their second dose, according to guidelines updated Tuesday by the Centers for Disease Control and Prevention. The agency is now proposing an eight-week interval between the first and second injections of those vaccines for some people over the age of 12, especially boys and men between the ages of 12 and 39.
The new change, which will not affect those already vaccinated, affects approximately 33 million unvaccinated people. And the CDC still recommends the original intervals — three weeks for Pfizer-BioNTech and four for Moderna — for certain people, including those whose immune systems are moderately to severely immunocompromised, are 65 years of age or older, or are at risk for severe Covid. .
The change comes in light of research showing that the longer interval between doses may increase the effectiveness of the vaccine and reduce the risk of a serious but uncommon side effect called myocarditis or inflammation of the heart. The overall incidence of myocarditis after vaccination is low. A study conducted in Israel estimated that nearly 11 out of every 100,000 men between the ages of 16 and 29 developed myocarditis.
Earlier this month, an independent panel of scientific advisors to the CDC reviewed research on myocarditis that supported an extended interval between doses. Research has shown that adolescent boys and young adult men have an increased risk of developing these heart problems after receiving their second dose of a Pfizer-BioNTech or Moderna vaccine.
Still, the absolute risk of myocarditis is very small, most cases are mild and short-lived, and research shows that a coronavirus infection is much more likely to cause heart problems than a Covid-19 vaccine. According to a CDC report, patients of all ages who were infected with the virus had nearly 16 times the risk of myocarditis than uninfected patients.
People who waited six to 14 weeks between the first two doses of Pfizer-BioNTech or Moderna vaccines had a higher antibody response than those who typically waited three or four weeks, according to a CDC report presentation from the meeting. And the effectiveness of the vaccines against infection and hospitalization was also higher with a six to eight week interval between doses.
When the vaccines were first rolled out, the priority was to fully vaccinate people as soon as possible, the presentation said. Now, more than a year into the US vaccination campaign, more data has become available to guide vaccination schedules, the presentation noted.
Public health experts say a longer interval between doses means there is a risk of people becoming infected after just one injection. But they say this change could affect some unvaccinated people and parents concerned about the potential risk of myocarditis.
During the panel meeting, Dr. Matthew Daley, a senior researcher at Kaiser Permanente in Colorado and a member of the advisory committee: “If the message is, ‘We already have a very effective and very safe vaccine or vaccines, and this is an approach to make them even safer’ , that might convince some people.”
Benjamin Mueller reporting contributed.