Here’s what you need to know about the new Omicron boosters.

The FDA has authorized updated boosters for fall, but questions remain about how well they work and how they will be used.

Doses of the original Moderna COVID-19 booster at a Southern Nevada Health District testing site in Las Vegas.Bridget Bennett/Bloomberg

For the first time during the COVID-19 pandemic, vaccine makers are getting ready to roll out updated versions of their shots. On Wednesday, the Food and Drug Administration cleared new shots from Pfizer and Moderna that are tailored to the Omicron BA.4 and BA.5 subvariants of the coronavirus, which account for more than 90 percent of new cases nationally.

The New York Times has reported that the Biden administration plans to make the shots available soon after Labor Day.

The Globe talked to several infectious disease and vaccine experts about what to expect.

How are the new boosters different?

The existing vaccines and boosters are based on the genetic code of SARS-CoV-2 from early 2020. The new boosters are “bivalent,” meaning they include the original formulation plus an updated version based on the genetic code of the Omicron BA.4 and BA.5 subvariants.

Dr. David Hamer, an infectious diseases doctor at Boston Medical Center, said the new boosters will “be great to have” to better combat the dominant variants. “The question is, will they continue [to circulate] or is something going to displace them over the next four to eight weeks?”

The hope is that the new boosters will provide better protection against the current strains, and maybe future ones as well, but it could take months to find out how well they work. Moderna just started a clinical trial of the vaccine and Pfizer said it will start one this month.

“The shots are very similar in structure and design to the ones that millions of people have already received, so they are likely to be comparably safe,” said Dr. Paul Sax, clinical director of the division of infectious diseases at Brigham and Women’s Hospital.

Why did the FDA review the shots before the clinical trials were finished?

Throughout the pandemic, vaccine manufacturers have designed shots for other variants, including Beta and Delta. But each time the firms finished testing the vaccines in clinical trials, a new variant emerged, making their latest design outdated.

This summer, Pfizer and Moderna finished testing a booster matched to the Omicron BA.1 subvariant, but by that time, the BA.4 and BA.5 subvariants already had started to spread. In an attempt to end the seemingly unending game of variant whack-a-mole, the FDA told the companies in late June to start making vaccines that matched BA.4 and BA.5.

The FDA said it would consider mouse data on the newest boosters, as well as human data from the BA.1 clinical trials, when making its decision to authorize the vaccines.

Many researchers said the FDA’s decision made sense. “It’s not uncommon for updated vaccines to rely solely on animal data. That’s often how the annual flu vaccine is updated,” said Jason L. Schwartz, an associate professor at the Yale School of Public Health.

But other scientists said the FDA should have required at least some testing in humans. “If nothing else, it would be a way to provide more confidence to the public,” said Michael T. Osterholm, a professor at the University of Minnesota and director of its Center for Infectious Disease Research and Policy.

How much better will the new boosters be?

“It’s not a question that we can easily answer,” said Dr. Richard Ellison III, professor of medicine at the UMass Chan Medical School and hospital epidemiologist at UMass Medical Center.

Since the existing boosters provide good protection against hospitalization and death, Ellison expects the new boosters will, too. But the degree of protection, particularly against mild illness and infection, will depend on how much the virus continues to change, he added.

Moderna and Pfizer said their BA.1 boosters — recently authorized in the United Kingdom — triggered higher levels of neutralizing antibodies against that subvariant than the original booster. But the trials were not large enough to determine if those higher antibody levels translated to reduced rates of infections, hospitalizations, and deaths. “Those are things that are revealed only after the vaccines come into wide use,” Schwartz said.

Without clinical data on the new boosters, it’s impossible to know if they will be better than their predecessors, said Dr. Shira Doron, infectious disease physician and hospital epidemiologist at Tufts Medical Center. “We should not be dishonest and oversell what we can expect it to do.”

Who should consider getting the new booster?

The FDA has authorized Pfizer’s updated vaccine for people ages 12 and older and Moderna’s in adults 18 and older. The Centers for Disease Control and Prevention is likely to make recommendations about who should get the shots after its advisory panel meets on Thursday and Friday.

Doctors say the new booster will be most helpful for people who are at the highest risk of developing severe COVID. That includes people with diabetes, heart disease, kidney disease, or obesity, as well as people over 65, Ellison said.

People who haven’t gotten any booster shots yet should “definitely” consider getting the new one, Sax said, as well as high-risk individuals who got their last booster 6 to 12 months ago. “If they received their last shot in late 2021 or early 2022, then they definitely should get the new one,” Sax added.

If you were going to get a booster soon, should you wait for the new one?

Although most public health experts and doctors encourage people to get their shots as soon as possible, many suggested that, in this case, people should wait until the updated boosters are available.

“The only possible exception are people who see themselves engaging in activities that put them at very high risk of COVID in the next couple of weeks. They may want to get their booster now,” Sax said.

Should people who were recently infected with Omicron get the new booster?

Researchers hope the FDA and CDC will provide specific guidance for people who have already been infected with Omicron, especially the most recent BA.5 subvariant. “These are things that we need our public health specialists to weigh in on, because there have been tens of millions of Omicron infections this spring and summer,” Schwartz said.

Sax thinks that the booster shot will be helpful for previously infected people, but that they can probably wait three to six months after infection before getting the shot. “People who had COVID recently can definitely put off getting the booster,” he said.

Dr. Sabrina A. Assoumou, an infectious disease doctor at Boston Medical Center, said. “It is possible to get another Omicron infection if you’ve been infected earlier in the year.” She thinks boosters could help reduce cases this fall and winter.

So when will the new shots be ready?

Now that the FDA has authorized the shots, the CDC’s group of independent vaccine experts, known as the Advisory Committee on Immunization Practices, or ACIP, is meeting on Sept. 1 and 2. During the pandemic, the group has often convened right after the FDA authorized a COVID vaccine or booster to make recommendations about who should get the shots.

Pfizer and Moderna said they would be ready to start shipping the updated vaccines in September, although it is likely that supplies will be limited at first. It’s also unclear what will happen to the many existing boosters already stocked at vaccination sites across the country.

Dr. Daniel Kuritzkes, chief of the infectious diseases division at Brigham and Women’s Hospital, said people shouldn’t be discouraged by the uncertainties and debates over the best strategies for boosting.

“The FDA, the manufacturers, and the CDC are doing their best in a nearly impossible situation of trying to predict what’s going to happen in the future, and make the best decision for public health with a very limited data set,” he said.

This article was updated on Aug. 31 to indicate that the FDA has authorized the Omicron booster shots.