Federal health officials have greenlighted a second shot of the omicron booster for some older adults and those who are immunocompromised.

On Wednesday, advisors to the Centers for Disease Control and Prevention met and expressed support for allowing certain high risk groups to get a second shot of the updated COVID-19 vaccine — known as the "bivalent" vaccine because it targets both omicron and the original strain of the virus.

Following the meeting, the CDC issued a statement endorsing the new recommendations, which align with the Food and Drug Administration's recent decision to authorize a second shot. For now, the opportunity to a boost of the bivalent vaccine will be limited to those age 65 and older who got their first shot at least four months ago, and to those with weakened immune systems who got one of those shots at least two months ago.

Along with allowing a second shot for some adults, the CDC is also working to streamline its COVID-19 vaccine recommendations: Now most people are considered "up to date" if they've had one shot of the most current vaccine.

For the time being, that will be the bivalent vaccine, which was released in the fall. However, drugmakers are expected to release a new formulation of the COVID-19 vaccine — one that better matches the strains that are circulating — later this year.

The CDC's new recommendations mean that it no longer matters how many shots someone had in the past. Even those who had no prior shots would be considered "up to date" if they had one shot of the bivalent vaccine or whichever vaccine becomes available in the future.

"FDA has consistently over-interpreted the performance of the bivalent formulation when given as a booster. Now it seems to have gone beyond the science and decided it has some kind of magic power as a first dose," John Moore, an immunologist at Weill Cornell Medical College, wrote in an email to NPR.

Is a second shot worth it?

Less than 17 percent of those eligible for the bivalent shot have opted for one, and so the demand for another right now would probably be even lower.

The change will bring the U.S. more in line with the approach taken by Britain and Canada, which is what some vaccine specialists have been urging — especially with the ample of supply of vaccines available.

"Those doses are going to be expiring and will be thrown out. So it makes sense to have those shots in arms instead of being tossed in the waste basket," said Dr. Peter Hotez, co-director of the Texas Children's Hospital Center for Vaccine Development and dean of the National School of Tropical Medicine at Baylor College of Medicine.

The main concern is that the protection people got from their last shot has been fading, not just against getting infected but also possibly against getting seriously ill. So Hotez says people as young as 50 should be able to get a second bivalent booster if they want one.

"It's better than nothing," Hotez wrote in an email to NPR . "I think 65 could be lowered to 50 or 55 unless they have specific data that only supports that age cutoff."

He added: "Historically, when you look at the monovalent vaccines, the protection starts to wane after four or five months. We don't know if that's the case with bivalent boosters. But you don't want to find out the hard way."

But others scientists aren't so sure. They say there just isn't any good evidence showing protection against serious illness has faded significantly or that getting another shot would help that much.

"I have no data to show me that a second bivalent is safe and effective. I have every reason to think it might be. But I don't actually have data," said Dr. Gregory Poland, director of the Mayo Clinic's Vaccine Research Group.

In addition, there's a theoretical possibility that it could kind of backfire — because the bivalent boosters target a strain that's already been replaced by a new one called XBB.1.5.

"The concern is that if we continue to give boosters against a virus that's not circulating when we do see the next variant you may not develop a vigorous immune response to that new viral variant," Poland said.

But some people say they will rush to get one when they can.

"I'm very pleased that I don't have to lie!" wrote Annie Bristow, 73, of Frostburg, Md., in an email to NPR. She had been considering telling a pharmacist a "white lie" to get another booster.

Others are disappointed they'll still be ineligible, including Ellen McDaniel-Weissler, 63, who lives in rural Maryland.

"I am deeply convinced that the COVID pandemic is not over in spite the fact that people are suffering from COVID fatigue as am I," said McDaniel-Webster. "But people are still dying of COVID every day."