As doses of the updated Covid-19 booster shots become available, health experts are encouraging eligible individuals to get boosted ahead of potential surges this fall and winter—but many have differing opinions on when people should get the reformulated dose.

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Background

FDA last week authorized updated Covid-19 boosters from Pfizer-BioNTech and Moderna ahead of the United States' expected fall booster campaign. Pfizer-BioNTech's booster is authorized for individuals ages 12 and older, while Moderna's booster is authorized for all adults 18 and older.

On Thursday, CDC's Advisory Committee on Immunization Practices (ACIP) recommended both booster shots—Pfizer-BioNTech's for people 12 years and older and Moderna's for people 18 years and older. Following ACIP's recommendation, CDC Director Rochelle Walensky also endorsed the decision—a move that cleared the way for some providers to start administering the shots.

The boosters are bivalent vaccines, which target both the original coronavirus variant and omicron subvariants BA.4 and BA.5.

"As we head into fall and begin to spend more time indoors, we strongly encourage anyone who is eligible to consider receiving a booster dose with a bivalent Covid-19 vaccine to provide better protection against currently circulating variants," said FDA Commissioner Robert Califf.

According to Peter Marks, head of FDA's vaccines division, the updated vaccines are expected to better match future variants that could evolve this winter. "In terms of trying to stave off serious outcomes and symptomatic disease, one needs to refresh the immune system with what is actually circulating," he said.

FDA controversially evaluated the boosters' safety and efficacy using studies involving mice, as well as human neutralizing antibody data from previous studies looking at bivalent BA.1 boosters. But, according to Walensky, "This recommendation followed a comprehensive scientific evaluation and robust scientific discussion."

The updated boosters should provide a level of protection that was not possible with the original formulation of the vaccine. However, some experts suggest that how well the shots work may be dependent on an individual's past exposure to the virus—whether through vaccination, infection, or both.

"A bivalent vaccine will have some benefit for almost everybody who gets it," said Rishi Goel, an immunologist at the University of Pennsylvania. "How much benefit that is, we're still not exactly sure." 

'There is no bad time to get your Covid-19 booster'

While CDC has recommended booster shots for all individuals ages 12 and older, experts have a range of opinions on who needs an updated booster dose and when they should get it.

For instance, CDC guidelines say that people are eligible for a booster dose at least two months after their last Covid-19 shot, whether it is a booster or an initial vaccine. However, some experts have said it would be better for these individuals to wait at least four months.

"What we've seen is that almost everybody who is eligible for a boost is far more beyond two months from their last shot," Walensky said. "Certainly we wouldn't want somebody to get a boost too soon, and we wouldn't want you to get a boost before two months. But I would say if you're three, four, five months after your last shot, now is the time to go ahead and get it."

"I will get it," said Bob Wachter, chair of medicine at the University of California, San Francisco, and a physician who is in his mid-60s and in good health. "I'm about eight months out from shot number four. And so my immunity has waned significantly."

"There's no question that getting a booster increases the likelihood that you'll have a benign case," if you are infected, he added.

Wachter said he agrees with CDC's recommendation that younger adults also get a booster dose. According to Wachter, booster shots can help protect people from long Covid and can help protect the community by lowering transmission.

"There are good reasons to get it, even for people that have a low chance of a super severe infection," Wachter said.

Monica Gandhi, an infectious disease expert at the University of California, San Francisco, said she "would recommend this booster shot for those who are immunocompromised or those who are 60 years [old] and above," adding that people in these groups are at highest risk.

However, many experts have noted that individuals who have had a recent Covid-19 infection should not get a booster right away. In particular, CDC vaccine advisors recommend delaying a booster shot by three months for people with recent Covid-19 infections.

Judith Guzman-Cottrill, an infectious disease specialist at Oregon Health & Science University, and her children had mild Covid-19 infections last month, so she said she will wait until November before she gets her booster shot.

"Our natural antibody response will protect us against COVID for another few months. So I do think it makes sense to wait and get the updated booster about three months after our positive COVID test," Guzman-Cottrill said.

While experts are anticipating fall and winter surges, many have advised against delaying a booster dose until this time. According to Wachter, this method could be compared to trying to time the stock market—it is difficult to predict exactly when a surge of cases will come, so waiting for a booster could increase risk.

"You are basically accepting a period of vulnerability that you don't need to have," Wachter said. "And as I weigh all that, my thinking is I'd rather not do that."

Separately, Aniruddha Hazra, an infectious disease specialist at the University of Chicago, noted that it could be risky to delay a booster until a surge has already begun. "It does take a few weeks for our immune systems to be primed," Hazra said.

Ultimately, Walensky said, "[i]f you are eligible, there is no bad time to get your Covid-19 booster and I strongly encourage you to receive it." (Treisman, NPR, 9/2; Aubrey, "Shots," NPR, 9/5; Gardner, Politico, 9/1; Sun, Washington Post, 9/1)