Millions of Americans vaccinated against the coronavirus have the green light to get a booster dose — a shot aimed at fortifying their defenses against a highly transmissible variant that ignited a fourth deadly wave of illness.

The actions this week of two of the nation’s major health agencies open the newest chapter in the 18-month campaign to stop the coronavirus but not without tensions.

In a rare move, CDC Director Rochelle Walensky late Thursday overruled her agency’s advisory panel and added a recommendation for boosters for people whose jobs put them at risk of infection.

The advisory panel had made a recommendation that largely mirrored an authorization issued a day earlier by the Food and Drug Administration, with a call for a third shot of the Pfizer-BioNTech vaccine for people 65 and older, nursing home residents and people 50 to 64 with underlying medical conditions six months after completing their second shot.

The panel also said younger people, 18 to 49 years old with underlying medical conditions, may assess their own risk and choose to get a booster if they want one.

The CDC panel parted with the FDA on a pivotal issue: It declined to recommend a booster for people at risk of illness because of their job.

On Sept. 24, President Biden urged front-line workers, Americans over 65 and those with underlying health conditions to get to get a booster shot. (The Washington Post)

While Walensky agreed with most of what the advisers recommended, she added a recommendation that hewed closely to the FDA language. Individuals ages 18 to 64 at risk of exposure and transmission because of occupational or institutional setting may receive a booster shot, she said in a statement, released just before midnight.

The FDA had authorized Pfizer boosters for several populations: people 65 and older; those 18 to 64 at high risk of severe illness; and those 18 to 64 years whose “frequent institutional or occupational exposure” to the coronavirus puts them at high risk of covid-19 complications, including teachers, health-care workers and grocery store employees.

Walensky said in a statement it was her job to “to recognize where our actions can have the greatest impact.”

“At CDC, we are tasked with analyzing complex, often imperfect data to make concrete recommendations that optimize health,” Walensky said. “In a pandemic, even with uncertainty, we must take actions that we anticipate will do the greatest good.”

“I believe we can best serve the nation’s public health needs by providing booster doses for the elderly, those in long-term care facilities, people with underlying medical conditions, and for adults at high risk of disease from occupational and institutional exposures to COVID-19.”

She said that while the vaccine advisory panel reviewed data only for the Pfizer-BioNTech vaccine, the CDC “will address, with the same sense of urgency, recommendations for the Moderna and J & J vaccines as soon as those data are available.”

The recommendations of the panel are not binding, but it is rare for a director to overrule the committee.

The question about occupational exposure drew heated debate among the CDC advisers and was voted down, 9 to 6.

Pablo J. Sanchez, a pediatrician at Ohio State University, voted against it because he said it would open the door to anyone who wanted to get a vaccine.

“We might as well just say give it to anyone 18 and over,” Sanchez said. “We have a really effective vaccine, and it is like saying that it is not working, and it is working.”

Others, including Grace Lee, a pediatrician at Stanford University School of Medicine and chair of the panel, voted in favor of the recommendation, saying it would provide access to people who think they need the booster.

Walensky endorsed the other recommendations of the Advisory Committee on Immunization Practices, making them official and paving the way for tens of millions of adults to get a third Pfizer shot six months after completing the original two-shot regimen. With the CDC approval, the shots are expected to be available immediately at locations where the Pfizer vaccine is already being administered, including pharmacies, health departments, clinics and some doctor’s offices.

Several panel members said they thought the data did not necessarily support a booster for the general population, noting that the current vaccination regimen — two shots of the mRNA vaccines or one dose of the Johnson & Johnson product — continues to provide strong protection against hospitalization and death. They also noted that the best way to “move the needle” in curbing the pandemic is not with boosters but with getting the unvaccinated their first shots, said Helen Keipp Talbot, an infectious-diseases physician at Vanderbilt University.

Gerald E. Harmon, president of the American Medical Association, praised the CDC panel for its “thoughtful deliberations and recommendations.” With hospitalization rates 10 to 22 times as high among the unvaccinated as among the vaccinated, he said in a statement, “our top priority should remain reaching those individuals who remain unvaccinated against covid-19.”

The AMA awaits further reviews of the evidence on booster doses for “adults at risk of occupational or environmental exposure, such as health care personnel, teachers, and other essential workers,” which “we believe could help preserve our nation’s health care capacity so we can continue providing care to our patients.”

But some clinicians reacted with swift criticism of the advisory panel’s vote against recommending boosters for people at risk because of their jobs. Some doctors and public health officials pointed out that many health-care workers and teachers would not be included under the advisory panel’s recommendations. Some also noted the difference of opinion with the FDA.

Criticism on social media was immediate.

Jonathan Reiner, a D.C. cardiologist, tweeted: “So a 25 yo smoker can get a booster but not an ICU nurse? The CDC ACIP committee is disconnected from reality. I suggest the members spend some time rounding in a COVID ICU and then decide whether it makes sense to withhold boosters from healthcare workers vaccinated 9 mos ago.”

Clay Marsh, West Virginia’s coronavirus czar, said he was surprised the panel’s recommendations didn’t include booster shots for health-care workers.

“Right now our nation’s hospital system — at least some areas — are being pushed to the absolute breaking point,” Marsh said. The problem is not just one of bed capacity but also of staff shortages, he said.

Some administration officials expressed frustration about the committee decisions. They wanted the booster recommendations to be as expansive as possible and to cover workplace exposure, which they said can pose significant risks to employees, regardless of age.

“In a pandemic you want really clear, crisp recommendations,” said one official, who spoke on the condition of anonymity because he was not authorized to discuss the issue. “This is like a free-for-all.”

As new coronavirus variants emerge and the longevity of vaccine protection remains unknown, scientists are researching how booster shots could work. (John Farrell/The Washington Post)

The CDC panel provides clinical guidance for a vaccine’s use in medical practice. The FDA’s role addresses whether a vaccine is allowed to be used in certain populations. Certain pneumonia vaccines are allowed by the FDA to be used in people ages 19 and up, but the advisory panel does not recommend their use because it is not necessarily effective, said Jason Goldman, a Florida physician and liaison member to the advisory panel. The HPV vaccine is approved for a much greater age range than what the advisory panel recommends.

In addition, from a practical standpoint, consumers who want a booster will almost certainly find ways to get one. Consumers are not required to get a prescription or a letter from their doctor. About 70 percent of vaccine doses are being given at pharmacies, and boosters will not require additional documentation, CDC officials said.

“In reality, anyone who wants a booster will get one, as has already been happening,” said one federal health official who spoke on the condition of anonymity because they were not authorized to comment publicly.

Administration officials on Thursday said they were privately hoping the CDC and its advisers would make a recommendation similar to, or at least compatible with, the FDA’s authorization so that the government did not deliver differing, confusing messages on who should get the booster and when.

The recommendations released Thursday are interim, and panel members may amend them in the weeks ahead when more data become available, including information about the ability to mix and match vaccines, CDC officials said.

In remarks to the panel before the vote, Walensky sought to include those whose jobs put them at higher risk, such as a “35-year-old pregnant resident physician, working in a Tennessee ER, and with a 1-year-old at home.”

Because the FDA authorization pertains only to third doses of the Pfizer-BioNTech vaccine, and federal guidance encourages consumers to get boosters from the maker of their original doses, many policy questions remain about implementation. State health officials, hospital administrators and clinicians need guidance now, they say, on whether they can mix and match booster shots from the two other authorized vaccines, Moderna and Johnson & Johnson.

Nirav Shah, director of Maine’s Center for Disease Control and Prevention, said officials face major logistical hurdles because the same staff members are responsible for giving boosters, providing annual flu shots and preparing for pediatric coronavirus vaccinations when those vaccines receive regulatory clearance. The inability to mix and match vaccine products, he said, “poses significant operational challenges.”

Federal officials have suggested clearance of booster shots for the Moderna and Johnson & Johnson vaccines could happen in a few weeks.

An estimated 26 million people are at least six months past their second Pfizer dose, according to the CDC. About half are 65 and older. An additional 23 million people are at least six months past their second Moderna shot, and about 3.3 million people are at least six months past their one shot of the Johnson & Johnson vaccine.

The Biden administration had been hoping it could offer boosters beginning this week to any fully vaccinated adults eight months after their second shot, to reduce viral transmission and curb the pandemic. That broad plan ran into opposition from outside experts and some of the government’s own vaccine scientists, who argued that two doses of the vaccines still offer robust protection against serious illness and hospitalizations, even if some data indicates waning protection against less serious cases.

Federal health officials have already authorized a third shot of the Pfizer and Moderna vaccines for people with weakened immune systems, such as cancer patients and organ transplant recipients.

Rachel Roubein contributed to this report.