When to get your Omicron booster, and other things to know about the new shots

Here’s what to know about the new shots, now that the FDA and CDC have signed off.

This August 2022 photo provided by Pfizer shows vials of the company's updated COVID-19 vaccine during production in Kalamazoo, Mich.Associated Press

More than 200 million Americans are now eligible to get new booster shots updated to match the Omicron BA.4 and BA.5 subvariants of the coronavirus, which account for the vast majority of new COVID-19 cases nationally.

On Thursday, following authorization by the Food and Drug Administration, the Centers for Disease Control and Prevention recommended that people as young as 12 receive the new boosters this fall.

The long-awaited vaccines fulfill a promise made by Moderna and Pfizer to use their mRNA vaccine technology to rapidly update their shots to match whatever versions of the coronavirus are circulating.

Pfizer and Moderna said they would be ready to start shipping the updated vaccines in the coming days, although supplies will be limited at first.

Who should get the new boosters?

The new Pfizer shots are authorized for people 12 and older, and the Moderna booster for those 18 and older.

The FDA said people could get the Omicron boosters as soon as two months after their most recent COVID shot or booster, although some scientists say that waiting longer between shots may help bolster the immune response. Several studies over the past year have suggested that spacing out the COVID shots may lead to better immunity.

People who haven’t gotten any booster shots yet should “definitely” consider getting the new one, as well as high-risk individuals who got their last booster 6 to 12 months ago, said Dr. Paul Sax, clinical director of the division of infectious diseases at Brigham and Women’s Hospital. “If they received their last shot in late 2021 or early 2022, then they definitely should get the new one,” he added.

“If you are eligible, there is no bad time to get your COVID-19 booster and I strongly encourage you to receive it,” CDC Director Rochelle P. Walensky said in a statement.

With the authorization of the Omicron boosters, the FDA revoked its previous authorization of the original booster shots for people 12 and older. Pfizer plans to ask the FDA to authorize the Omicron booster for children ages 5 to 11 in early October, and will keep the original booster available for that age group until the new one is available to them.

Do people who were recently infected with Omicron need the new booster?

The CDC has previously said people “may consider delaying” their next vaccine dose three months after testing positive for COVID-19 or developing symptoms of the disease.

Sax thinks 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.

“Even if a person has had the infection, their immunity wanes over time with each passing month, so it’s worth getting the booster,” said Dr. Phillip Landrigan, an epidemiologist and the director of the Global Public Health Program at Boston College.

What’s different about the new boosters?

The original 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 Moderna and Pfizer just recently began clinical trials of the vaccines, and it will take months to find out how well they work.

“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,” Sax said.

Why did the FDA authorize the shots without testing them in clinical trials?

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 and CDC considered mouse data on the newest boosters as well as human data from the BA.1 clinical trials when making their decisions in favor of the shots.

Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, said in statement that the updated vaccines “meet our rigorous safety, effectiveness and manufacturing quality standards.”

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 Yale’s Schwartz.

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.

A panel of vaccine experts convened by the CDC voted 13 to 1 on Thursday in favor of recommending the new boosters, suggesting that waiting for clinical data could lead to more hospitalizations and deaths from the virus. The sole objector, Dr. Pablo Sanchez from The Ohio State University, said he wanted to see data from clinical studies in humans before recommending the new booster.

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 original 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.

At the CDC meeting on Thursday, Moderna said its new vaccine triggered about 4.5 times as many neutralizing antibodies against BA.5 in mice compared to the original booster, and Pfizer said its new vaccine spurred about 2.6 times as many antibodies. Moderna also showed that the new booster was better at reducing viral replication in the lungs.

This article used material from previous Globe stories. Camille Caldera contributed to this report.