The F.D.A. authorized second boosters for Americans aged 50 and older and those with certain immune deficiencies on Tuesday.

But the scientific evidence for the additional dose is incomplete, at best, and researchers do not agree on whether the shots are needed. So should you get one? My colleague Apoorva Mandavilli answered some common questions.

What do scientists think about second boosters?
Many are dubious. Experts point out that the limited research so far supports a fourth shot only for those older than 65 or who have underlying conditions that put them at high risk.

The most compelling data comes from an Israeli study that found that adults older than 60 who got a fourth dose were 78 percent less likely to die of Covid than those who got only three shots — but that study is deeply flawed. The participants volunteered to get a fourth shot and therefore are likely naturally to be more careful about their health, experts said.

Other factors may make the booster shot look more effective than it actually is. In fact, other data from Israel suggest that a second booster has only marginal benefits in healthy young people.

Who benefits most from a second booster?
Probably people who are immunocompromised or older than 65, according to the limited evidence available.

“If you’re more than five or six months out from your last booster, and you’re at high to very high risk,” the obvious choice is to get the shot, said Dr. Robert Wachter, chair of the Department of Medicine at the University of California, San Francisco. “As a healthy 64-year-old man whose third shot was seven months ago, I will get one this week if I can.”

Should I get the booster now? Or time it to my summer vacation or the next surge?
It takes the immune system about a week to rev up after the shot. From that peak, antibodies taper down over the next two to three months — so a booster shot now will not offer much defense in August or even July. If you qualify for a booster, you may want to have the maximum protection at the right time, like during a trip or a surge.

“The one thing that matters is where we are in this whole pandemic,” said Marion Pepper, an immunologist at the University of Washington. “I would be watching what the variants are doing.”

Are there drawbacks to getting a second booster?
Maybe. Repeated boosting offers diminishing results. Pepper’s team has evidence suggesting that a fourth exposure to the virus — whether through infection or the vaccine — will not make immunity any stronger than it is after the third.

There is also some worry that repeatedly boosting with the original version of the vaccine will make the body less responsive to future versions. There is some evidence that more spacing between shots may produce a stronger, more lasting immune response.

I just had Covid. Do I still need another booster shot?
Probably not for a while, all the experts said in interviews. Pepper’s team found that in people who had been infected and then vaccinated, even the first booster shot did not offer much of an advantage.