The U.S. Food and Drug Administration this week is expected to authorize the first updated versions of the COVID-19 boosters since the pandemic began.

The new shots are reformulated versions of the Moderna and Pfizer-BioNTech vaccines. They're known as "bivalent" vaccines because they are designed to protect against the original strain and the highly contagious omicron variant.

Specifically, the vaccines are programmed to target the BA.4 and BA.5 omicron subvariants, which are the dominant strains infecting people and the most adept at sneaking around the immune system.

The hope is the shots will bolster peoples' waning immunity and provide stronger protection against catching the virus, spreading it and getting sick with COVID and long COVID.

The Biden administration is planning to start making the new shots available after Labor Day to help blunt the impact of what could be yet another surge of infections this fall and winter.

"This is a really important moment in this pandemic," Dr. Ashish Jha, the White House COVID-19 Response Coordinator told NPR. "This is the first major upgrade of the vaccines — first major change in the vaccines — in the last two and a half years."

But the formulation of the boosters and the process for authorizing them has sparked debate among scientists.

For the first time, the FDA is judging how well the vaccines work without results from tests done directly in people. To save time, the FDA is initially evaluating the vaccines with tests in mice along with the results of tests that were done on people of an earlier version of a bivalent vaccine.

Some experts worry that mouse studies aren't very reliable at predicting how well vaccines work in people.

"It could be problematic if the public thinks that the new bivalent boosters are a super-strong shield against infection, and hence increased their behavioral risk and exposed themselves to more virus," says John Moore, an immunologist at Weill Cornell Medicine.

But federal officials defend the decision.

The mouse studies suggest the new vaccines may be about 20 times more protective against omicron than the original shots, and about five times more protective than the first attempt to create omicron-specific bivalent vaccines, Dr. Peter Marks, director of the Center for Biologics Evaluation and Research at the FDA, told NPR in an interview.

"That makes us feel confident that they will do what they are intended to do, which is to produce a good immune response against the BA.4/5 variant, as well as refresh our overall response given the original component of the vaccine as well," Marks says.

The decision to rely on mouse studies became necessary after the FDA in June rejected new boosters that targeted the original strain of omicron, known as BA.1, and instead asked the vaccine companies to develop new shots targeting the strains that had replaced it.

Some scientists think there's the possibility that the new shots could also give people immunity that lasts longer than the original shots, and maybe even protect against new variants that emerge. But more research is needed to confirm that.

Some experts say the data from the BA.1 boosters indicate any potential improvement could be pretty modest at best.

"We want a silver bullet. And the booster has become the silver bullet. And we're putting all our eggs in the vaccine basket," says Dr. Celine Gounder, a senior fellow at the Kaiser Family Foundation. "I am very skeptical as to how much of an improvement these vaccines will yield in terms of population immunity and prevention of severe disease."

Gounder also worries that the country has given up on doing anything else to protect people, like wearing masks and improving ventilation.

But others are more optimistic about the new boosters.

"I personally am very excited about the bivalent vaccines," says Jenna Guthmiller, an assistant professor of immunology at the University of Colorado.

"We really need an updated vaccine to provide protection against the current omicron lineage viruses as well as potentially any future omicron variants," Guthmiller says. "I think it's going to be good."

After the FDA authorizes the vaccines, advisers to the Centers for Disease Control and Prevention will meet Thursday and Friday to decide whether to recommend it and who should receive it. CDC Director Dr. Rochelle Walensky will then have to sign off on that recommendation.

Some experts says only people who are at high risk because of their age or underlying health problems need to get another booster since the first shots are still protecting most people against severe disease. Others say everyone age 12 and older who hasn't been infected or boosted recently should get a new shot.

"I would say that anyone who is longer than six months since their previous boost or previous infection should go get a boost," says E. John Wherry, an immunologist at the University of Pennsylvania.

"Any opportunity to get more boosters into the population to increase vaccine uptake is going to be a positive thing in helping us get through this pandemic," Wherry says.

The Biden administration has purchased more than 170 million doses of the the new boosters, which should start to become available after Labor Day.

It remains unclear how much of a demand there will be for the new boosters, given that many eligible people still haven't gotten their first or second boosters.