(NEXSTAR) – Booster shots targeting the most common strains of the COVID-19 omicron variant are arriving at pharmacies across the country this week.

The new round of booster shots marks the first time the FDA has approved an update to the COVID-19 vaccine. The new boosters combine the original vaccine released in December 2020, and protection against the omicron versions BA.4 and BA.5.

Who should get a booster shot?

Updated shots made by Pfizer and its partner BioNTech are authorized for anyone 12 and older, and rival Moderna’s version is for adults. They’re to be used as a booster for anyone who’s already had their primary vaccination series — using shots from any U.S.-cleared company — and regardless of how many boosters they’ve already gotten.

There are some restrictions when it comes to who can take them, however. You have to have received the full, initial series of COVID vaccine shots, and the FDA recommends waiting two months after your last vaccine or booster dose before getting the new booster. Other experts, including advisers to the Centers for Disease Control and Prevention, say waiting at least three months – and even six months if the person doesn’t have pre-existing health conditions – can be beneficial.

“If you wait a little more time, you get a better immunologic response,” said CDC adviser Dr. Sarah Long of Drexel University.

That’s because someone who recently got a booster already has more virus-fighting antibodies in their bloodstream. Antibodies gradually wane over time, and another shot too soon won’t offer much extra benefit, explained John Wherry, a University of Pennsylvania immunologist who wasn’t involved with the government’s decision-making.

Where can I get one?

A number of local hospitals and pharmacies are offering the shots free of charge right now. Here are a few of the current options, but supply levels and appointments may vary:

If you want to look for something closer to home, you can always check the White House’s Vaccine.gov site to find a local provider.

A number of states, including Massachusetts, New Jersey, Colorado and California, have their own websites as well to link up residents with clinics and other vaccine providers.

How helpful will the boosters be?

That’s not clear, because tests of this exact recipe have only just begun in people.

The FDA cleared the new boosters based in large part on human studies of a similarly tweaked vaccine that’s just been recommended by regulators in Europe. Those tweaked shots target an earlier omicron strain, BA.1, that circulated last winter, and studies found they revved up people’s virus-fighting antibodies.

With that earlier omicron version now replaced by BA.4 and BA.5, the FDA ordered an additional tweak to the shots — and tests in mice showed they spark an equally good immune response.

There’s no way to know if antibodies produced by an omicron-matched booster might last longer than a few months. But a booster also is supposed to strengthen immune system memory, adding to protection against serious illness from the ever-mutating virus.

As has been the case with COVID-19, there is still a possibility of a markedly different variant popping up this winter.

Dr. Anthony Fauci, the nation’s top infections disease doctor, said this week that “all bets are off, and we change,” if that happens, but noted that we’ve had “almost a year now of omicron sublineages” and that the boosts “have done pretty well” in protecting Americans from serious illness and death.

“If we continue to have an evolution of what we used to call and still do call an “influenza adrift” — not a major change, but just sort of drifting along to BA.5 sublineage — I believe that that would fit in well with what we’re talking about the likelihood that we’ll get into a cadence, that on a yearly basis for most people we’ll be able to cover what is out there as the dominant variant,” Fauci said.

The Associated Press contributed to this report.