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The Centers for Disease Control and Prevention recommended Thursday that millions of eligible Americans, including those as young as 12, get an updated omicron-targeting booster shot to bolster defenses against serious illness and death during a potential fall or winter rise in COVID-19 cases.

CDC Director Rochelle Walensky endorsed a recommendation by an advisory panel, paving the way for some clinicians, pharmacies and other providers to begin administering the shots as early as this weekend. The Advisory Committee on Immunization Practices voted 13-1 to recommend updated shots from Moderna, for those 18 and older, and from Pfizer-BioNTech, for people 12 and older.

Thursday’s action, along with authorization of the shots a day earlier by the Food and Drug Administration, marked another turning point in the pandemic and reflected the persistent struggle to tamp down illness and death 2 1/2 years since the pandemic dawned.

“The updated COVID-19 boosters are formulated to better protect against the most recently circulating COVID-19 variant,” Walensky said in a statement. “They can help restore protection that has waned since previous vaccination and were designed to provide broader protection against newer variants. . . . If you are eligible, there is no bad time to get your COVID-19 booster and I strongly encourage you to receive it.”

Several advisory panel members expressed concern about the lack of clinical data about the reformulated boosters but also noted the potential harm in waiting for clinical data until November.

Matthew Daley, a physician at Kaiser Permanente Colorado, said the cost of waiting until later in November to roll out the updated booster shot could be an additional 9,700 deaths and 137,000 hospitalizations, based on projections presented at the daylong meeting.

“I think that is the tension that I feel for sure,” Daley said. But with the FDA decision, “we’re now in the position where we have millions of doses of bivalent vaccines that are ready and available. And I think they’re going to be an effective tool for disease prevention this fall and into the end of the winter.”

The latest recommendation for boosters provides a new opportunity for the Biden administration and public health experts to launch another round of messaging to a pandemic-weary public about the importance of vaccination against a virus still killing an average of more than 500 people a day in the United States.

Doses began shipping Wednesday to pharmacies, clinics and doctor’s offices after the FDA authorized the updated shots from Moderna and from Pfizer and its German partner, BioNTech. Like earlier coronavirus vaccines, the updated boosters will be free.

The new boosters — the first changes since the mRNA vaccines were rolled out in December 2020 — target the BA.4 and BA.5 omicron subvariants dominant in the United States. Officials say the new shots will help broaden immunity because they are a closer match to the strain that is circulating.

Until now, vaccines have targeted the original version of the coronavirus, even as different variants emerged. Half of the new booster, known as a bivalent vaccine, contains the original formulation, while the other half carries a recipe against BA.4 and BA.5, by far the most contagious versions of the virus since coronavirus swept the globe in 2020. BA.5 now accounts for nearly 90% of cases in the United States, according to the CDC.

People can receive the updated shots if it has been at least two months since they completed their primary vaccine series or their most recent booster. Even if an individual received boosters of the original formulation, they can get the updated booster as long as it has been two months after their last shot. The interval of at least two months is intended to broaden immunity because getting shots too early reduces the vaccine’s effectiveness.

A longer interval between shots also lessens the risk, especially for young adults and older teens, of rare side effects such as myocarditis, inflammation of the heart muscle, health officials have said.

The CDC estimates about 200 million Americans 12 and older are eligible for the updated shot. While nearly 22 million adults 50 and older have received a second booster dose, most people 5 and older are at least six months out from their last coronavirus vaccine dose, CDC official Sara Oliver told the advisory panel Thursday.

Americans have been slow to get boosters, and experts say it is unclear whether uptake of the reformulated booster will be different. The country may be transitioning from the coronavirus as an emergency, “to becoming something we have to learn to manage on a more regular, routine basis, including through routine [coronavirus] immunization,” said Jen Kates, a senior vice president at the Kaiser Family Foundation.

“It’s hard to see how there will be a groundswell, a rush to get boosted,” Kates said in an email. Some people — about 20% of people who have been vaccinated — “are waiting for updated vaccines that can target variants,” Kates said, referring to a recent Kaiser Family Foundation survey. “On the other hand, if people don’t perceive these as better or needed, or have other reasons for not wanting to get a booster, it’s hard to see how this will change things.”

Albert Ko, an infectious-disease physician and epidemiologist at the Yale School of Public Health, said he was worried about what Americans will face this winter, noting the country is experiencing more than 500 COVID-19 deaths a day.

“Poor uptake of the new boosters is a real and urgent concern since we rely on vaccination to protect our communities, especially now that the use of social distancing and face masks is disappearing,” Ko said.

Panel members, echoing questions among some experts, raised concerns about the lack of human data on the shots’ effectiveness — the FDA relied heavily on mouse studies — which experts have said could fuel skepticism about the boosters’ effectiveness. The data used by the FDA to authorize the shot included human studies of earlier experimental bivalent shots, including one that generated virus-fighting antibodies against BA.1 — the first omicron subvariant — and the overall record of the shots since December 2020.

Pablo J. Sanchez, professor of pediatrics at Ohio State University, said he voted no because he thinks that “we need the human data” on the new vaccine, which is only being gathered now.

But Jeffrey Duchin, health officer for Seattle and King County, said he was comfortable with the animal data in support of the reformulated boosters. Panel members also noted that animal studies have been regularly used to adjust the composition of annual influenza vaccines.

Oliver, of the CDC, presented data showing how a booster vaccination program in September could avert significant deaths, hospitalizations, infections and direct medical costs. Oliver noted that as the virus has evolved, the effectiveness of vaccines has waned more rapidly. Inclusion of a variant in the vaccine broadens the antibody response.

Interest in earlier booster shots has proved lukewarm, at best.

In a recent Kaiser Family Foundation survey, almost 6 in 10 people who are vaccinated but not boosted said they feel they have enough protection, “which we know from the data is not the case given waning and new variants,” Kates said. Three in 10 said they are just too busy, and 15% are concerned about missing work, the survey found. Democrats are more likely to have been vaccinated and boosted than Republicans.

A CDC survey conducted in August with the University of Iowa suggests more people will want the shots — 72% of eligible respondents said they definitely or probably will get an updated booster against omicron, Oliver said.

Only half of booster-eligible Americans — about 108 million people — received the first recommended booster dose, and only about a third of those 50 and older — about 22 million people — got a second booster. CDC surveys have shown that older adults, college graduates and people with higher incomes remain most likely to be vaccinated and boosted.

Anyone who has received the two-shot primary series of the mRNA or Novavax vaccines or the single-shot Johnson & Johnson vaccine will be eligible, regardless of whether they received any booster shots.

FDA officials expect pediatric data on the new boosters in the next month or two and could authorize the shots for some children younger than 12 later this year.

The booster change is already causing confusion. Some people who signed up to receive the original booster formulation will need to reschedule appointments to get the new version because the original formulations are no longer authorized by the FDA to be used as boosters.

“The last thing we need to be doing is telling people who signed up [for the original booster] and expected to get vaccinated this week that they need to wait,” said Michael Fraser, chief executive of the Association of State and Territorial Health Officials. “We have seen lagging demand nationally, and this doesn’t build confidence among the public and providers who are asking their health official what to do now.”

The CDC recommends coronavirus vaccination for everyone 6 months and older, and boosters for everyone 5 years and older who is eligible.

CDC data at Thursday’s meeting showed that adults who had a primary series and two boosters had a 14 times lower risk of death from COVID-19 than unvaccinated people. People who had a second booster had a threefold lower risk of death than people who had only one booster.

Experts and officials have differed on whether an updated booster is needed because the original vaccines still offer strong protection against severe disease and death for generally healthy people, especially if they received the first booster dose. Some experts have said it is not clear how much additional benefit a reformulated booster will offer.

People should check with local pharmacies and providers and visit vaccines.gov before showing up to receive shots because only about 1.5 million doses are likely to be available initially, according to information provided to state health officials. Another 10 million doses are scheduled to be delivered next week.

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The Washington Post’s Laurie McGinley contributed to this report.