In a nod to the ongoing risk the coronavirus poses to millions of Americans, the Centers for Disease Control and Prevention recommended on Wednesday that adults 65 and older and those with weakened immune systems receive another dose of the reformulated booster that debuted last fall.
The endorsement followed a daylong discussion by the C.D.C.’s expert advisers. The Food and Drug Administration authorized the booster plan on Tuesday, and the C.D.C.’s recommendation was the final administrative step. Eligible Americans will be able to receive booster doses immediately.
Federal health officials are also phasing out the original vaccine formulas created by Pfizer-BioNTech and Moderna, revoking their authorizations in the United States. And instead of needing an initial series of two shots, unvaccinated people will now require just a single dose of the reformulated, or “bivalent,” Covid shot to be considered vaccinated.
Until now, federal officials had required two doses of the older vaccine before recipients could begin to receive the bivalent boosters, a process some experts felt was confusing.
Limited data on the reformulated vaccines indicate that in older adults, the shots offer additional protection against severe disease and death from Covid, although the protection wanes rapidly in the weeks after inoculation.
There are about 53 million adults 65 and older in the United States, accounting for about 16 percent of the population, according to the Census Bureau. And seven million Americans have weak immune systems because of an illness or a medication.
Roughly 250 people in the United States are still dying from Covid-related causes each day, a vast majority of whom are over 70 or have impaired immune systems. The median age of those hospitalized is 75, according to the C.D.C. Yet only about 43 percent of adults 65 and older have received a bivalent booster shot so far.
By this point, most Americans have built up some immunity against the virus, whether through prior infections, vaccinations or both. The new guidelines acknowledge as much, but allow for those still at high risk from the virus to protect themselves, and to do so free of charge.
“The one-size-fits-all policy was simple but not optimal,” said Dr. Jeremy Faust, an emergency medicine physician and health policy expert at Brigham and Women’s Hospital in Boston. “The new regimen acknowledges that there’s now an extraordinary spectrum of Covid risk, from mild to massive, depending on who you are.”
People who are severely immunocompromised, such as organ transplant recipients, may want to opt for booster shots every six months or even more frequently, Dr. Faust said.
The C.D.C. now says that adults 65 and older may opt for another dose of the bivalent vaccine at least four months after their first shot. Those with weakened immune systems may do so two months after their previous bivalent dose, and may choose to receive more doses in consultation with their health care provider.
In the meeting of C.D.C. advisers on Wednesday, Dr. Camille Kotton, a physician at Massachusetts General Hospital, noted that the new recommendations did not include immunocompromised children 6 months through 4 years of age. That leaves those medically frail children — including recipients of organ transplants — unprotected against the virus, she said.
“As a mom and doctor, this seems like we are leaving them so vulnerable,” she said in an interview.
For most Americans, the F.D.A. plans to encourage annual Covid shots in the fall, much as it does with flu shots. Health officials will decide on the exact composition of the shot in June, based on the version of the virus circulating at the time.
The bivalent vaccine targets the original variant of the coronavirus as well as variants BA.4 and BA.5, which were dominant last summer. But BA.4 and BA.5 were quickly supplanted by newer versions.
The most recent Omicron subvariant, XBB.1.5, now accounts for about 78 percent of cases in the United States, and another version, XBB.1.16, for about 7 percent. So far, the reformulated vaccines seem to prevent severe illness and hospitalization after infection with these variants.
Federal health officials also are making changes to the process for those receiving the initial shots.
Unvaccinated people will receive a single dose of the bivalent vaccine, rather than multiple doses of the original monovalent vaccine. The rationale is that most unvaccinated Americans now presumably have some measure of immunity from a prior infection and may not need two doses at the beginning, the F.D.A. said.
Some experts were sharply critical of the advice. Reams of data suggest that the vaccines are most protective when given in two doses and followed by one or more boosters to reinforce the shield, said John Moore, a virologist at Weill Cornell Medical College.
“F.D.A. has consistently over-interpreted the performance of the bivalent formulation when given as a booster,” Dr. Moore said. “Now it seems to have gone beyond the science and decided it has some kind of magic power as a first dose.”
It may be reasonable to assume that nearly all unvaccinated adults have already been infected at least once and can get by with just a single dose, said Deepta Bhattacharya, an immunologist at the University of Arizona.
“I guess the F.D.A. is just trying to simplify given the reality on the ground,” he said. But “immunologically, you’d want to get two shots if it’s your first exposure.”
The agency could instead recommend two doses of a bivalent vaccine and say that those who know of a prior infection can elect to forego the second dose, Dr. Bhattacharya said. But, he added, “in reality, I doubt a clause like that would make much practical difference.”
The F.D.A. said it had “carefully reviewed the available epidemiologic evidence, scientific publications and data provided by sponsors indicating that a single bivalent vaccine dose provided to individuals previously infected with Covid-19 provides an immune response equal to, or superior to, two doses of the original vaccine.”
“The agency believes that this approach will help encourage future vaccination, particularly among those who have not chosen to be vaccinated to date,” the F.D.A. said in its statement.