WASHINGTON >> Older Americans and people with weak immune systems can get an extra COVID-19 booster dose this spring.

The Centers for Disease Control and Prevention on Wednesday signed off on a more flexible booster schedule for people who remain at the highest risk from COVID-19 — giving them the choice of a second “bivalent” Pfizer or Moderna booster, the most up-to-date formula.

“Many in the population are experiencing vaccine fatigue but there is a subset who are eager to receive additional doses,” CDC’s Dr. Sara Oliver told an agency advisory panel that expressed support for the change.

The move came a day after the Food and Drug Administration took steps to make coronavirus vaccinations simpler for everyone. From now on, anyone getting a Pfizer or Moderna dose — whether it’s a booster or their first-ever vaccination — will get an updated version rather than the outdated original shots.

Here are some things to know:

WHO NEEDS A BOOSTER?

Anyone who’s gotten their original vaccinations but hasn’t had an updated booster yet can still get one. Only 42% of Americans 65 and older — and just 20% of all adults — have gotten one of those updated boosters since September.

WHO CAN GET A SECOND UPDATED BOOSTER?

People 65 or older who already had one Pfizer or Moderna updated booster can roll up their sleeves again as long as it’s been at least four months since that last shot.

The schedule is a little different for people with weak immune systems. Most can choose a second Pfizer or Moderna updated booster at least two months after their first. Under the latest FDA and CDC guidelines, they also could get additional doses if and when their physician decides they need one.

WHY THE EXTRA LEEWAY?

Older adults continue to have the highest rates of hospitalization from COVID-19, even as cases have declined. But a frail 85-year-old may want another booster right away while a robust 65-year-old may not see the need — or might instead time another shot for peak protection ahead of a summer vacation or other special event.

CDC officials stressed there’s even more variety among immune-compromised patients, from people with only mild impairment to those trying to replenish immunity that grueling cancer treatment knocked out.

The changes put the U.S. in line with Britain and Canada, which also are offering certain vulnerable populations a spring shot. It’s a reasonable choice, Dr. Matthew Laurens, of the University of Maryland School of Medicine, said before the announcement.

“We do have vaccines that are available to protect against these severe consequences, so why not use them?” he said. “They don’t do any good just sitting on a shelf.”

WILL YOUNGER, HEALTHIER PEOPLE GET A FALL DOSE?

Stay tuned. The FDA will hold a public meeting in June to consider if the vaccine recipe needs more adjusting to better match the latest coronavirus strains — just like it adjusts flu vaccines every year. And part of that discussion will be whether younger, healthier people also need a booster.

The updated Pfizer and Moderna shots being used now target the BA.4 and BA.5 omicron versions, which have been replaced by an ever-changing list of omicron descendants. Still, while protection against mild infections is short-lived, those updated doses continue to do a good job fighting severe disease and death even against the newest variants.

TOTS ARE LEAST LIKELY TO BE VACCINATED YET

CDC’s advisers were dismayed at how few of the youngest children are vaccinated. Just 6% of 2- to 4-year-olds have gotten their initial COVID-19 shots and 4.5% of those younger than 2. Far fewer got an updated booster.

The FDA’s new rules mean tots under 5 who’ve never been vaccinated can get the most up-to-date formula — two Moderna shots or three of the Pfizer-BioNTech version. Unvaccinated 5-year-olds can get two Moderna doses or a single Pfizer shot. And tots already fully or partially vaccinated may get a bivalent shot or two depending on their vaccination history.

WHAT ABOUT THE NOVAVAX VACCINE?

Novavax makes a more traditional type of COVID-19 vaccine, and its original formula remains available for people who don’t want the Pfizer or Moderna option. Novavax also is getting ready in case FDA urges a fall update, by manufacturing several additional formulas.