The Biden administration has approved a fourth Covid-19 vaccine shot for all Americans over age 50 and for all adults who are immunocompromised. But does that mean everybody who is eligible should rush out to their pharmacy or primary care doctor to get it?

The short answer is that it depends — on both your personal risk and what’s happening with the pandemic. Making things even more perplexing, the public health guidance has become more nuanced as more booster shots are authorized. Whereas public health experts were unified in urging people to get their first and second shots last year, they were more divided about third shots when those were approved late last year, at least until the emerging omicron wave made the first round of boosters more urgent.

With fourth shots now getting the okay from federal officials, public health experts are emphasizing that individual circumstances should dictate whether or not you should get another booster shot right away. Some experts are skeptical that more shots are really necessary for people except for perhaps the older age groups (people over 65, for example) and for people with certain immune system conditions. Others say simply that the evidence is mixed and people will have to make their own assessments, in consultation with a doctor.

“I don’t think it is a right or wrong answer with a clear overwhelming evidence one way or another,” Alessandro Sette, an immunologist at the La Jolla Institute for Immunology, told me. “That maybe makes it a bit confusing for everybody.”

For people with conditions that compromise their immune response, Sette was unequivocal: Yes, go get your shot as soon as you’re eligible. (The CDC has said people should wait four months between doses.) Those people have tended to see less of a benefit from the vaccines so far and, as the Washington Post reported, part of the reason the Biden administration is pushing ahead with a fourth shot is to provide protection for people in the event of another Covid-19 wave created by the omicron BA.2 variant.

For otherwise healthy people over 50, it becomes more of a personal choice. Sette himself is over 50 and therefore eligible. “I’m glad I have the option,” he said. “I’m not rushing to get it.”

Other countries have limited their fourth shots to people much older than 50 — over 70 in Germany and over 75 in the United Kingdom, for example, along with immunocompromised people in both countries. The risk of having serious Covid-19 complications rises with age, so experts say getting another booster is most urgent for the oldest age cohorts, especially in case the new omicron BA.2 variant were to drive another wave of infections across the US.

“This will give some people who got boosters early on more protection against BA.2 than they have currently,” Bill Hanage, a Harvard University epidemiologist, told me over email. “Is it necessary? Not for younger people. Older people? Quite possibly.”

For people closer to 50, it may be more a matter of personal taste and risk tolerance. Monica Gandhi, an infectious disease doctor at UC San Francisco, pointed to a study of health care workers that found no evidence of a fourth shot providing more protection or decreasing viral loads for people under 65. With that in mind, she said she would advise a fourth shot only for patients older than 65.

Preliminary research out of Israel did find that a fourth shot reduced the risk of death for people over 60. But it should be remembered, Sette pointed out, that any improvement from a fourth shot is starting from a very high baseline. The three-shot protocol recommended by health authorities had already proven very effective at preventing the most serious outcomes for most people.

And preventing more hospitalizations and deaths, many experts say, should be the primary goal of the vaccines, as opposed to preventing any infections at all.

The human body’s two-tiered immune response makes the former much easier than the latter. When a person gets vaccinated, they have a surge in antibodies, which can prevent a Covid-19 infection from taking hold at all. But over time, those antibodies, that first layer of defense, will start to wane. Even with booster shots, the evidence indicates that the boost in antibodies is only temporary.

But that doesn’t mean a person is fully vulnerable to the coronavirus if they contract it again. Their body still has memory cells that can recognize Covid-19 and start to produce antibodies if the person gets infected. That process may not happen quickly enough to prevent an infection. But it should, for most people, be able to prevent their symptoms from becoming so severe that they wind up in the hospital.

For the elderly and immunocompromised, there is a stronger case for trying to keep antibody levels high because they tend not to see as strong of an immune response from vaccination in the first place. We don’t necessarily want to risk an infection taking hold in those people because there are cases of vaccinated older people or people with immune conditions contracting Covid-19 and getting very sick or even dying.

Hanage argued that, in order to prevent more serious illness and death, it is most important to get people who haven’t yet had a third dose their first booster shot. The United States has lagged behind its peers like the UK in getting even its vulnerable population, such as people over 65, a third shot.

“While fourth shots have value in these populations, it would be even better to get third shots into vulnerable folks who have not yet received them,” he said.

And for younger and healthier people, they can be more confident that the protection they have received from three shots is resilient.

Sette said he would probably get a fourth shot eventually, but he may wait to get it until closer to a family vacation he has planned for the summer. Experts interviewed by the New York Times also said people might want to consider their plans before getting another shot, timing the dose to maximize protection when they may be most exposed because of travel, for example.

The one variable that would make another booster more urgent for everybody would be a new surge taking off or a new variant emerging. In that case, Sette said, the case for getting a fourth shot as soon as possible would be stronger.

“This can change on a dime,” he said.