Tens of millions of Americans can sign up to get Moderna and Johnson & Johnson boosters beginning Friday after the nation’s top public health official endorsed recommendations from expert advisers that the shots are safe and effective at bolstering protection against the coronavirus.

The green light from Rochelle Walensky, director of the Centers for Disease Control and Prevention, means that eligible Americans at risk of severe disease can choose any of the three boosters now authorized in the United States regardless of their original shot.

“The evidence shows that all three COVID-19 vaccines authorized in the United States are safe — as demonstrated by the over 400 million vaccine doses already given,” Walensky said in a statement Thursday night, several hours after receiving unanimous recommendations from the expert panel, called the Advisory Committee on Immunization Practices. “And, they are all highly effective in reducing the risk of severe disease, hospitalization, and death, even in the midst of the widely circulating delta variant.”

Walensky’s action — following authorization Wednesday from federal regulators — largely fulfills the administration’s August pledge to make boosters of all three vaccines available to Americans, albeit a month later than promised and for a smaller group. The administration’s focus on boosters came as the highly contagious delta variant sickened millions and killed tens of thousands, and also reflected concern about waning immunity from the vaccines.

CDC’s sign-off on the additional boosters as well as the flexibility to mix and match the shots gives greater leeway to consumers, as well as to clinicians and pharmacies administering them to vulnerable populations. Health officials have repeatedly sought ways to make it easier for people to get a booster dose, especially those who have had side effects from one brand, or who worry about risks associated with a particular shot.

The availability of boosters will be particularly welcome to the 15 million recipients of the Johnson & Johnson vaccine, many of whom have been particularly fearful of breakthrough infections given that shot’s lower level of protection compared with the messenger RNA vaccines.

“I agree that those who received a [Johnson & Johnson] vaccine should receive a second dose — I would prefer that those individuals get an mRNA vaccine” rather than a second Johnson & Johnson shot, said advisory panel member Pablo J. Sanchez, a pediatrician at Ohio State University.

Interchangeability of shots is also likely to speed booster vaccination in nursing homes and other institutional settings where residents received different shots during the early rollout. The Pfizer-BioNTech booster is already in use since it was authorized and recommended last month.

“I think the opportunity for these [mix and match] boosts [is] priceless,” said Helen Keipp Talbot, an infectious-disease doctor at Vanderbilt University and panel member.

The CDC plans to release guidance early next week with more detailed information about who might benefit from choosing one booster over another, as the panel requested. CDC advisers and agency officials are still working out whether to recommend that some people stick to their original vaccine if possible.

“A really important aspect of all of this is being clear and not dancing on the head of a pin so that we don’t confuse the American people,” said Beth Bell, a global health professor at the University of Washington.

The advisory panel’s recommendation was similar to Wednesday’s action by the Food and Drug Administration. The FDA did not take a position on whether people should stay with the original vaccine or switch to another one, saying it did not have the data to make such judgments.

The FDA has authorized a third shot of Moderna or Pfizer-BioNTech for anyone 65 and older, or any adults at high risk of severe illness because of underlying conditions, job exposure or because they are in institutional settings, and who have gone at least six months since their second dose.

It broadened eligibility much further for those who received the single-shot Johnson & Johnson vaccine to anyone 18 and older who has gone at least two months since getting the shot — criteria reflecting the lower protection afforded by that vaccine compared with the others.

Advisers to the CDC suggested in their all-day meeting Thursday that mixing and matching booster shots may appeal to consumers concerned about possible risks associated with their first vaccine.

“A lot of what our efforts are centered around is trying to mitigate risk as much as possible, both from disease as well as vaccination,” said Grace Lee, a professor of pediatrics at Stanford University School of Medicine and chair of the panel.

More data on the safety of booster shots for specific groups may help determine “whether or not a different boost would be appropriate, for example, for young women” who first received Johnson & Johnson, Lee added.

Some panel members questioned the wisdom of administering a second Johnson & Johnson shot to women of childbearing age, for instance, because of rare but serious risk of blood clots associated with that vaccine. Analyses of data for those who have received that shot suggest an increased risk of a rare type of clot, especially for women 18 to 49 years old.

The advisers and the CDC do not recommend getting a Johnson & Johnson booster for anyone who developed blood clots after getting a first shot.

Both Moderna and Pfizer vaccines are also associated with the extremely rare risk of inflammatory heart problems, such as myocarditis, especially in males ages 12 to 30. Some safety data from the United States, Canada and Scandinavian countries show greater risk with Moderna than Pfizer, but others show no difference between the brands, according to a presentation by Talbot, who heads the CDC panel’s vaccine safety group.

Follow-up study of the heart problems suggests the cases are generally mild and symptoms resolve promptly, Talbot said. The Moderna shots tied to that risk were full doses, not the half-dose booster cleared by the FDA and recommended by the CDC panel. Safety data of the half-dose Moderna booster is limited, but the risks might be lower at that lower dose, Talbot said.

For people who develop myocarditis after vaccination with Moderna or Pfizer, and who are eligible for boosters, the advisers and the CDC recommend deferring the shot at least until symptoms have completely resolved.

About 105 million fully vaccinated people have received the two-shot Pfizer series, according to the CDC. About 70 million fully vaccinated people have received the Moderna shots. Only 15 million Americans were vaccinated with Johnson & Johnson shots, which arrived later and were delayed by an investigation of a rare adverse event, as well as a manufacturing problem.

More than 11 million people have received a booster or an additional dose of a vaccine to date.