In a sweeping victory for the Biden administration, a scientific advisory panel to the Centers for Disease Control and Prevention on Thursday endorsed booster shots of the Moderna and the Johnson & Johnson Covid-19 vaccines for tens of millions of Americans.
The decision follows an agency endorsement last month of booster shots of the Pfizer-BioNTech vaccine. If the director of the C.D.C., Dr. Rochelle Walensky, signs off on the newest recommendations — as she is expected to — Americans could seek out a booster shot as early as Friday.
Her approval would bring the country closer to fulfilling President Biden’s promise in August to offer boosters to all adults. The pandemic is now retreating in most parts of the country, but about 75,000 people are diagnosed with the disease every day, and about 1,500 die from it.
That pledge angered many experts, including some advising the Food and Drug Administration and the C.D.C., who said that scientists had not yet had a chance to determine whether boosters were actually necessary.
Studies showed that the vaccines remained very effective against severe disease and death, although their effectiveness might have waned against milder infections, particularly as the Delta variant spread across the nation this summer.
The purpose of the vaccines is to prevent illness severe enough to require medical attention, not to prevent infection, Dr. Wilbur Chen, an infectious disease physician at the University of Maryland and a member of the C.D.C. panel, the Advisory Committee on Immunization Practices, said during the deliberations on Thursday.
“It might be too much to ask for a vaccine, either a primary series or the booster, to prevent all forms of infections,” Dr. Chen said.
The C.D.C.’s advisers last month tried to narrow the number of Americans who would be eligible for a booster dose of the Pfizer-BioNTech vaccine, saying that research did not support boosters for people whose jobs exposed them to the coronavirus, as the F.D.A. had indicated.
But in a highly unusual move, Dr. Walensky overturned their decision, aligning the agency’s advice with the criteria laid out by the F.D.A.
On Wednesday, the Food and Drug Administration authorized booster shots for millions of people who received the Moderna and the Johnson & Johnson vaccines, just as it did for recipients of Pfizer-BioNTech shots last month. The F.D.A. also gave the green light for people eligible for booster shots to get their dose of a different brand from the one they first received.
But in practice, who will get the shots and when depends greatly on the C.D.C.’s final guidance. Though the agency’s recommendations do not bind state and local officials, they hold great sway in the medical community.
On Thursday, members of the C.D.C.’s panel endorsed the so-called mix-and-match strategy, saying people fully immunized with one company’s vaccine should be allowed to receive a different vaccine for their booster shot.
Limited evidence strongly suggests that booster doses of one of the two mRNA vaccines — Moderna or Pfizer-BioNTech — more effectively raise antibody levels than a booster dose of the Johnson & Johnson vaccine.
The committee advised that recipients of the single-dose Johnson & Johnson Covid vaccine should receive a booster shot at least two months after their first dose.
Among Americans initially immunized with an mRNA vaccine, adults over 65, adults who are 50 to 65 with certain medical conditions, and those who reside in long-term care settings should receive a single booster dose six months or longer after their second dose, the committee decided.
For adults ages 18 to 49 with certain medical conditions and adults whose jobs regularly expose them to the virus, the panel opted for softer language, saying they may choose to get a booster after considering their individual risk.
The experts emphasized that people who have received two mRNA vaccine doses or a single Johnson & Johnson dose should still consider themselves fully vaccinated. Federal health officials said they would continue to study whether those who had weak immune systems and had already received a third dose of a vaccine should go on to get a fourth dose.
Some advisers were concerned that young and healthy Americans who don’t need a booster might choose to get one anyway. Side effects are uncommon, but in younger Americans they may outweigh the potential benefits of booster doses, the scientists said.
“Those that are not at high risk should really be thoughtful about getting that dose,” said Dr. Helen Talbot, an infectious disease expert at Vanderbilt University.
The committee’s final votes contrasted sharply with discussions earlier in the day. The panel heard that in adults under 65, even those with chronic conditions, the Moderna vaccine remained highly protective against severe illness and showed only a small decline in effectiveness over time, if any at all.
The Johnson & Johnson vaccine showed less efficacy than the Moderna vaccine overall, but the data were too limited to determine whether there might be a decline over time.
Having already authorized the Pfizer-BioNTech booster, however, some advisers said in interviews that they felt compelled to do the same for the other two vaccines, adding that it was only fair to people who had received those vaccines.
Just over 11 million people have opted for an additional shot so far, and up to three million make up those with weak immune systems who were approved to receive a third dose to prop up their immune response. Only 6 percent of people who are fully vaccinated, and about 15 percent of adults over 65, have received a booster dose so far.
Moderna’s booster shot will not be the same as its initial shot. The dose will be 50 micrograms, which is half the dose given in the initial rounds of immunization. Scientists from Moderna presented data indicating that the smaller dose is enough to rouse the immune system.
But the smaller dose may need to be delivered from the same vials now used for initial immunization. Some committee members noted that this may increase the risk of contamination and incorrect dosing. (Moderna has been testing vials that deliver smaller volumes of vaccine to alleviate this problem, according to a former government official.)
C.D.C. scientists said at the meeting that the Pfizer-BioNTech and the Moderna vaccines are generally safe, with the exception of uncommon and mostly mild heart problems in young men. The risk of the condition — called myocarditis, an inflammation of the heart muscle — is higher after the second shot of an mRNA vaccine, and highest in males 18 to 24 years old.
In those under 20, the condition may affect more than 100 males in every million immunized with an mRNA vaccine. Studies have shown that the risk of heart problems after a bout of Covid-19 is much higher.
The Johnson & Johnson vaccine carries a small risk of blood clots in young women. The company’s representatives said they had estimated the rate of blood clots at 15.1 cases per million after the first dose and 1.9 cases per million after the second.
Some panelists said they worried about the risk of blood clots in young women who get a second booster dose of the Johnson & Johnson vaccine, and the risk of myocarditis in young men after a third dose of an mRNA vaccine.
Perhaps young women should be directed to mRNA vaccines and young men to the Johnson & Johnson vaccine, Dr. Talbot said.
“We’re in a different place in the pandemic than we were earlier,” she said. The opportunities to mix and match vaccines “are priceless.”