The Biden administration has given the go-ahead for another COVID vaccine booster for people aged 50 and older and certain people who are immunocompromised. They can now get another Moderna or Pfizer-BioNTech booster at least four months after their last dose.

But just because you can get an additional booster, does that mean you need to?

Health officials argue that the protection provided by the COVID vaccine booster shots wanes over time. And they are concerned about people considered to be at highest risk of getting severe COVID.

But the Centers for Disease Control and Prevention didn't make it clear how urgently people should be lining up for second boosters. The agency says these groups are "eligible" for the shots but it stopped short of saying they should get them. And some infectious disease experts say not everyone in this age group needs another shot now.

So, if you're wondering whether to get a second booster, here are a few key factors to consider.

Risk of serious illness increases with age

Risk tracks with age, and older people have the highest risk.

A recent study among people 60 and older in Israel found that rates of COVID-19 infection and serious illness were lower in people who had a fourth dose of the Pfizer vaccine compared to three shots.

"We're talking about extra protection from the most serious outcome of COVID," says Dr. Eric Topol, founder and director of the Scripps Research Translational Institute.

Dr. Bob Wachter, chair of the Department of Medicine at the University of California San Francisco, says he personally plans to sign up for a second booster.

"I'm 64 and pretty healthy," he says. "But the evidence is clear that six months out from my first booster shot, the effectiveness of that booster has waned considerably."

He says another dose will boost his immunity and decrease the probability of infection. "The benefits are very real," Wachter says.

But for people under 60 it's less clear a second booster is necessary.

"I don't think we have the data for younger people, 50 to even 60," says Dr. Monica Gandhi, an infectious disease specialist at the University of California, San Francisco. The study out of Israel didn't include this younger age group.

She points out that other countries are targeting additional boosters for older people. Germany has authorized a fourth shot for people over 70. The U.K. is targeting people over the age of 75 and Sweden is giving fourth shots to people over 80. Gandhi says the U.S. "is jumping the gun" by forging ahead with shots for everyone over 50 without the relevant data.

Still the trendline is clear, says Dr. Peter Chin-Hong, an infectious disease specialist at UCSF.

"The older you are, the bigger the benefit," he says. Although the majority of deaths from COVID have been people older than 65, "there's a clear association with age and mortality with COVID," Chin-Hong says. "It's really, really striking and it starts at age 50."

His advice? "Walk to get the second booster if you're eligible." Then he says "walk a little faster the older you get." His mom is in her 80s and he wants to protect her as much as possible. "I'm telling her to walk quickly," he says.

Dr. Carlos del Rio, an infectious disease researcher at Emory University, thinks it's reasonable for people under 60 to wait. "The vaccines are holding up pretty well against severe disease and death," he says.

It's also worth noting that even for people over 60, the added protection of an additional booster shot is small in absolute terms. People who got the first booster already have a very low risk of dying from COVID. Chin-Hong points out that in the Israeli study less than .1% of people with a third shot died, a risk so low he calls it "remarkable."

Among people who got the fourth shot in this study just .03% died.

"Three shots is the magic number, we think, so far," he says.

Underlying conditions put you at higher risk

Certain medical conditions also increase the risk of serious illness and death from COVID-19 and that's the reason the FDA decided to authorize the additional boosters starting at age 50.

"We know that people in the age range from about 50 to 65 — about a third of them have significant comorbidities," said Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, at a press conference Tuesday. People with heart disease, lung disease, obesity and diabetes are at higher risk for serious illness and death and people over 50 — particularly people of color — are more likely to have an additional risk factor.

"So by choosing age 50 and up, to consider those at high risk or higher risk," Marks said. "We felt like we would capture the population that might most benefit from this fourth dose."

When it comes to age, "there's no bright cutoff of risk," agrees Wachter.

There are likely incremental increases in risk, year after year, as a person ages. A 50-year-old typically has lower risk than a 65-year-old, but health status matters, too.

"An unhealthy 55-year-old is probably at the same risk as a healthier 65-year-old," Wachter says.

Bottom line, risk goes up with age and underlying conditions, and Wachter says many people over 50 may benefit from another dose.

"Anyone who has a serious medical condition, I would certainly suggest thinking about getting a booster," says Dr. 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 says. "Because that extra layer of protection does help ensure that if they get COVID, it's going to be milder."

Immunocompromised people may need an extra boost

Health officials are particularly concerned about people who are immunocompromised because their immune responses to the vaccine tends to wane faster and they are at higher risk of getting severely ill or dying from COVID-19.

That's why anyone 12 or older with certain immunocompromised conditions can now get an additional shot of the Pfizer-BioNTech COVID-19 vaccine, four months after their last dose. A second booster of the Moderna vaccine can be given to people 18 years of age and older.

This includes people who have undergone solid organ transplants, or who are living with conditions that have a similar level of immunocompromise.

Timing from last dose or infection is important

There is mounting evidence of waning vaccine protection against serious illness from COVID-19 in older and immunocompromised people, who are at least four to six months past their first booster.

Evidence of waning immunity comes from a recent CDC analysis of COVID-19 emergency room visits and hospitalizations visits during the omicron-predominant period. Two months after a third dose, people were 91% protected against hospitalization. But by four months, that protection dropped down to about 78%.

"It means that people who were boosted three, four, five, six months ago probably have limited protection against current infection," Malani says.

This means a second booster can help shore up that protection, "but it's not going to be long lasting." So the timing of the additional shot can be tricky.

Right now the rate of viral infections has come down significantly since the peaks in January, but there are signs that infections are rising in some areas. The even more contagious omicron variant BA.2 is now the dominant variant in the U.S., and hospitalizations are also creeping up in some places.

Peter Chin-Hong says some people might want to wait to get a booster until a time when cases start to rise in their community and they need the added protection more urgently.

He also notes there may be more effective vaccines on the horizon. As vaccine makers test omicron-specific vaccines and continue research on vaccines that could fend off multiple variants, it may make more sense for people at lower risk to wait.

Still, if you're high-risk, you may not want to wait too long. Polls show many vaccinated people held off on a first booster dose when they became available last year. But waiting until you see another outbreak in your community could be risky.

"It reminds me a little bit of trying to time the stock market. It turns out nobody's actually good at it," Wachter says. If there's another outbreak on the horizon, it's best to maximize your protection in advance of it.

There's one more factor to consider when deciding on the timing of a fourth dose: Have you had a recent COVID-19 infection? If you've had three shots and you've had an omicron infection sometime between December and now, "I think it's reasonable to wait." Wachter says. He says a recent infection likely puts a person in a similar immunologic state as a second booster.

Rob Stein and Michaeleen Doucleff contributed to this report.

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