On Tuesday, FDA authorized, and CDC expanded eligibility for an additional Covid-19 booster shot for certain groups of Americans, but experts say whether a person needs another booster is dependent on a variety of factors.

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FDA and CDC authorize a second booster shot

FDA authorized a second booster shot for all adults ages 50 and over, as well as for other individuals with moderate to severe immunocompromising conditions and those "living with conditions that are considered to have an equivalent level of immunocompromise." Any individuals ages 12 and up meeting those criteria are eligible for an additional booster of the Pfizer-BioNTech vaccine, and those 18 years and older meeting those criteria are eligible for an additional booster of the Moderna vaccine.

Peter Marks, director of FDA's Center for Biologics Evaluation and Research, said FDA issued its authorization in response to data showing waning immunity and increasing risk of severe disease for certain groups. He added that FDA set its recommended age at 50 because that's when certain chronic conditions like heart disease and diabetes become more common.

"FDA believes this option will help save lives and prevent severe outcomes among our highest-risk patients," Marks said, adding that a second booster shot is especially important for the immunocompromised because they experience "more rapid waning of immune responses and they are generally more at risk for severe outcomes of Covid."

Later Tuesday, CDC expanded eligibility for a second booster shot to all Americans ages 50 and older as well certain people who are immunocompromised.

A second booster shot "is especially important for those 65 and older and those 50 and older with underlying medical conditions that increase their risk for severe disease from Covid-19 as they are the most likely to benefit from receiving an additional booster dose at this time," said CDC Director Rochelle Walensky.

White House officials told the Washington Post that the administration has enough doses to cover a second booster shot for all Americans ages 65 and older and primary doses for children under the age of 5, but can't place any additional vaccine orders unless Congress passes a funding package.

According to White House communications director Kate Bedingfield, President Joe Biden, who is 79, will receive a second booster shot if his doctor recommends it.

Who needs a second booster shot?

Experts say certain groups could benefit from a second booster shot, but exactly who would benefit depends on a number of factors.

Bob Wachter, chair of the University of California, San Francisco's (UCSF) department of medicine, said he agreed with FDA's decision, but added that people should consider their own situations.

When it comes to age, "there's no bright cut off of risk," Wachter said, adding, "An unhealthy 55-year-old is probably at the same risk as a healthier 65-year-old."

"For someone at high risk, it's a pretty clear call," Wachter said. "When you get to a low-risk 50-year-old, it's a little closer."

"Anyone who has a serious medical condition, I would certainly suggest thinking about getting a booster," said Preeti Malani, an infectious disease professor at University of Michigan Health.

"For my own family, for my parents and my in-laws, this is something that I will recommend," she added. "Because that extra layer of protection does help ensure that if they get Covid, it's going to be milder."

Kathryn Edwards, scientific director of the Vanderbilt Vaccine Research Program, noted that data on second booster shots is limited so far, though initial findings from Israel "suggest that a fourth dose increases the antibody responses." However, she added it's unclear how much immunity wanes and how quickly after that booster shot.

"I'm not against it, certainly, and I probably will as an old woman go get a fourth dose," Edwards said. "But it would be nice to have a little bit more information."

Monica Gandhi, an infectious disease specialist at UCSF, said she doesn't believe there's enough data "for younger people, 50 to even 60" to recommend a second booster shot, adding that other countries are authorizing boosters for older people.

Germany, for example, has authorized an additional booster for those over 70, while the United Kingdom authorized one for those over 75 and Sweden is providing additional boosters to those over 80. The United States "is jumping the gun" by authorizing second boosters for anyone over 50 without more data, Gandhi said.

Timing of a second booster is also important, according to William Moss, a vaccine expert at the Johns Hopkins Bloomberg School of Public Health. "The 'when' is a really difficult part," he said. "Ideally, we would time booster doses right before surges but we don't always know when that's going to be."

A longer time interval between doses also helps the immune system develop a stronger defense, the Associated Press reports.

"If you get a booster too close together, it's not doing any harm—you're just not going to get much benefit from it," said John Wherry, an immunologist at the University of Pennsylvania.

And someone who has recently recovered from Covid-19 may not need a second booster shot right away, according to Wachter.

"You can conceptually think about an infection like a booster," he said. "Folks who did have an infection in the last three months are likely as protected as if they got a second booster."

Research conducted by Marion Pepper, an immunologist at the University of Washington, found that people who had Covid-19 and were then vaccinated didn't see much of an advantage with their first booster shot.

"You sort of plateau at some sort of ceiling of immunity with three exposures to the spike protein, whether it's through infection and vaccine or just vaccine alone," Pepper said.

Research from Israel has found that second booster shots were unable to increase antibody levels higher than they were after the first booster, Bloomberg/Washington Post reports. "It stands to reason that eventually you get to a point where there are diminishing returns," said Alessandro Sette, an immunologist at the La Jolla Institute for Immunology.

Either way, people should have the option to get another booster if they feel it's necessary, said Leana Wen, a professor at George Washington University. "Giving people the choice to have an added level of protection is where we should be at this point in the pandemic," she said. "To me, the decision of getting an extra booster dose is not much different from the decision to continue masking or to use rapid tests before getting together with people indoors." (Walker, MedPage Today, 3/29; Doherty, Axios, 3/29; Neergaard/Perrone, Associated Press, 3/29; Joseph, STAT News, 3/29; Jarvis, Bloomberg/Washington Post, 3/29; Godoy et al., "Shots," NPR, 3/30; Mandavilli, New York Times, 3/29; Vakil, The Hill, 3/29; Owens, Axios, 3/30; Diamond et al., Washington Post, 3/22)