U.S. regulators on Tuesday authorized another COVID-19 booster for people 50 and older, a step toward providing additional protections for the most vulnerable in the event of a resurgence of the coronavirus.

The FDA decision provides these people with a fourth dose of Pfizer or Moderna’s vaccine, which is at least four months after the previous booster.

So far, the FDA has only approved a fourth dose for those 12 and older Severely weakened immune systemThis particularly vulnerable group also gets an additional booster, a fifth shot, the agency said.

The latest expansion, regardless of people’s health status, presents additional opportunities for millions of Americans — the question is whether everyone who qualifies should rush out to receive it. The Centers for Disease Control and Prevention is expected to be involved.

The move comes at a time of uncertainty.After winter surge, COVID-19 cases have dropped to low levels Hyperinfectious omicron variantsCDC data shows that two doses of the vaccine plus a booster dose are still effective in preventing serious illness and death.

but an omicron sibling is causing a worrying surge in infections in Europe — and spread in the US — even if vaccinations have stalled. About two-thirds of Americans are fully vaccinated, while half of those eligible for the first booster immunization are not.

Pfizer has asked the FDA to clear the fourth shot for people 65 and older, while Moderna has asked for “flexibility” for all adults so the government can decide who really needs a shot.

There is limited evidence of how much benefit another booster can provide now. FDA made the decision without receiving input from its independent panel of experts wrestling How much data is needed to enlarge the lens.

Download the Modern Healthcare app to stay informed as industry news emerges.

For older adults and people with other health conditions, “there may be reasons to top up the gas tank a bit,” said E. John Wherry, an immunologist at the University of Pennsylvania who was not involved in the administration’s decision.

But while he encourages older friends and relatives to follow the advice, Wherry, 50 – who is healthy, vaccinated and boosted – isn’t about to get a fourth shot right away. With protection against serious illness still strong, “I’ll wait until it looks like I need it.”

No COVID-19 vaccine is as resistant to omicron mutants as it is to earlier versions of the virus. Also, protection against mild infections naturally wanes over time. But the immune system builds up a multi-layered defense that prevents the type of serious illness and death that is being held back.

The CDC recently reported that during a U.S. omicron wave, two doses were nearly 80 percent effective against needing a ventilator or death, while a booster boosted that protection to 94 percent. The vaccine was least effective (74%) in the immunocompromised population, the vast majority of whom had not received a third dose.

U.S. health officials are also concerned about Israel, where a fourth dose was prescribed to people 60 and older at least four months after the last shot during the omicron surge. Preliminary data published online last week showed some benefits: Israeli researchers tallied more than 328,000 people who received the extra vaccine, 92 of whom died, and 232 of the 234,000 who skipped the fourth dose.

What’s unclear is how long any additional benefits of another booster will last, and when you’ll get it.

Dr William Moss, of the Johns Hopkins Bloomberg School of Public Health, said: “‘When’ is a very difficult part. Ideally we would time the booster dose before the surge, but we don’t Always know when that will be.”

In addition, longer firing intervals help the immune system build stronger, more cross-reactive defenses.

“If the booster gets too close, it doesn’t do any harm — you don’t get much out of it,” Wherry said.

The latest booster expansion may not be the last: Next week, the government will hold a public meeting to discuss whether everyone will eventually need a fourth dose (possibly in the fall) of the original vaccine or an updated vaccine.

As for the newer vaccine, ongoing studies in humans—using omicron-targeted injections alone or in combination with the original vaccine—are underway. The National Institutes of Health recently tested monkeys and found “no significant advantage” to using a booster that only targeted omicron.