The nation’s top public health official has given the green light to “mix-and-match” booster shots to increase antibodies to fight the coronavirus.

Under the decision from Centers for Disease Control and Prevention director Rochelle Walensky, eligible people can get any of the three available booster doses, regardless of which shot they received first. The Food and Drug Administration authorized the additional shots Wednesday.

As cold weather and the holiday season approaches, boosters are especially important for older people and those with underlying conditions that put them at risk of severe infections.

“Winter is coming,” said Jeanne Marrazzo, director of the division of infectious disease at the University of Alabama at Birmingham. “We really want everybody to think about it like topping off your antibody levels, like topping off the tank before winter comes.”

Here’s what you need to know about mix-and-match booster shots.

• What does “mixing and matching” vaccines mean?

A patient who receives a second or third dose that is from a different manufacturer than the prior dose will be “mixing and matching” vaccines. Those vaccines are also called “heterologous.”

One example would be a person who got the Johnson & Johnson shot in the spring and receives a Moderna booster this fall. Someone who received two initial doses of the Moderna vaccine but opts for the Pfizer booster would also be mixing and matching.

• What are the FDA’s latest recommendations on booster shots and mixing and matching?

Walensky authorized the Johnson & Johnson booster shot for people older than 18, to be administered least two months after their first vaccine. The move addresses data that has shown the initial single-shot regimen offered lower protection compared to the initial two doses of the Moderna and Pfizer-BioNTech vaccines.

She also gave the green light to a half-dose of Moderna as a booster for anyone over the age of 65, those with underlying conditions that put them at high risk and front-line workers who could be exposed to the virus at work. Eligible patients can get that booster six months or more after the initial two-shot regimen of Moderna. (A Pfizer booster had already been authorized in September.)

Walensky did the same for “mix-and-match” booster shots for anyone over the age of 65, adults with underlying conditions that put them at high risk of serious COVID-19 infections and front-line workers over the age of 18. Those eligible for a booster do not have to get the same vaccine they have already received.

That means people over 65 or in a high-risk category who initially received Moderna or Pfizer can also get any shot as a booster six months after their second dose. The agency also authorized all individuals over the age of 18 who received one dose of Johnson & Johnson to receive a booster of any of the three vaccines if it has been more than two months since the first shot.

• Is mixing and matching safe?

So far, experts say there is no evidence that receiving two different coronavirus vaccines causes any harm. And a recent study presented to the CDC on Friday that tracked people who received mix-and-match boosters did not report any severe adverse events related to the vaccines.

But experts also note that the data on mix-and-match vaccines is limited to a small number of people, especially compared to the much larger studies that have shown each of the vaccines to be safe and effective separately.

“We know that, in general, all of these vaccines are extremely safe,” said David Dowdy, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health. “So there’s no reason to think that there would be any problem with (mixing and matching).”

• Why are officials considering mix-and-match vaccines?

Allowing people to receive any vaccine as a booster may be a boon for people eager to get an extra shot ahead of family gatherings this winter. The decision could make it easier to find a booster shot and give health care providers more flexibility in offering boosters to patients who previously received a different vaccine than the one they have in stock now.

“It just could potentially speed things up,” Marrazzo said.

Any booster shot, even if it is the same as your first vaccine, will increase the number of antibodies that can fight the coronavirus. But, in some cases, mixing vaccines may offer better protection, experts say. Early data has shown that following up a first dose of Johnson & Johnson with an mRNA vaccine – Moderna or Pfizer – provided significantly higher levels of neutralizing antibodies.

“It’s incredibly confusing for the public with all this swirling data and everything happening so fast,” said Monica Gandhi, an infectious-disease specialist and professor of medicine at the University of California at San Francisco. She said the advantage of mixing and matching is particularly evident for recipients of the Johnson & Johnson vaccine who get a boost of Pfizer or Moderna.

“But something has really become clear: The mixing really is most impactful when you have a DNA/adenovirus vaccine first followed by the mRNA vaccine,” Gandhi said.

For people who received either Moderna or Pfizer, experts say it is both safe and effective to stick to the same vaccine.

• I am fully vaccinated, but I had a breakthrough infection – do I need to get a booster?

The jury is still out on whether a COVID infection provides enough lasting immunity to forego a booster shot. An immune system that has to fight the virus certainly produces antibodies, but those antibodies may fade away with time just as those created by vaccines eventually do.

Still, some experts believe that people who have contracted the virus and were also fully vaccinated ultimately develop the strongest immunity – whether the infection comes before or after vaccination. “You have the broadest immunity in terms of protection against variants,” said Paul Offit, a vaccine expert at the Children’s Hospital of Philadelphia who serves on the expert committee that advises the FDA.

Others believe that getting a booster, even after fully recovering from COVID-19, can’t hurt.

“I think there is no evidence that there will be harm,” said William Schaffner, a professor of infectious diseases at the Vanderbilt University Medical Center in Nashville.

• Why are boosters effective?

Booster shots help train the body to fully respond to pathogens.

A booster shot assists the body in creating memory cells that are durable and are capable of fighting off pathogens for stretches of time, said Costi Sifri, director of hospital epidemiology at University of Virginia Health.

“(Boosters) are basically providing a second signal for the immune system to fully complete its response,” he said. “They hardwire the immune system to respond to that pathogen.”

With previous coronavirus variants, vaccine antibodies from the initial doses may have been good enough to protect against catching the virus for up to six months, Sifri said, but the more contagious delta variant has underscored the need for booster shots so the body can continue fighting against serious illness or hospitalization.

“You boost up those antibody levels where you’re able to prevent the delta from causing breakthrough infections,” he said.

Scientists are utilizing knowledge from other viruses and applying it to the coronavirus, but they’re still on the steeper end of the learning curve, said Tom Russo, professor and chief of infectious disease at the University at Buffalo Jacobs School of Medicine and Biomedical Sciences.

“We learned you need a tetanus shot every 10 years, but there was a point where we didn’t know that,” he said. “We’re learning on the fly with COVID”

The Washington Post’s Fenit Nirappil and Lena H. Sun contributed to this report.