Americans will now have access to updated COVID booster shots after the Centers for Disease Control and Prevention signed off on reformulated versions of the Moderna and Pfizer-BioNTech COVID-19 vaccines Thursday night.

Some doses could be available as soon as Friday, with a wider rollout planned for next week. Health officials expect another surge of infections this fall and winter, and say the shots — which target the original coronavirus strain as well as the more contagious omicron variant — will help boost peoples' waning immunity and protect against serious disease and death.

What should you keep in mind if you're ready to roll up your sleeve? CDC director Dr. Rochelle Walensky spoke with Morning Edition's Steve Inskeep about the new boosters.

"Doses are rolling into pharmacies and other sites now, and I would say if you're eligible for your boost there is no bad time to go out and get one," Walensky says.

There are eligibility and timing considerations

Adults 18 years or older can get the Moderna booster, while the Pfizer-BioNTech version has been authorized for people 12 and up. In both cases, a person is only eligible for a booster if it has been at least two months since their last COVID vaccine.

Some vaccine experts say that it would be better for people to wait until four months after their last COVID shot or infection for maximum efficacy, though Walensky suggests there is some gray area.

"What we've seen is that almost everybody who is eligible for a boost is far more beyond two months from their last shot," she says. "Certainly we wouldn't want somebody to get a boost too soon, and we wouldn't want you to get a boost before two months. But I would say if you're three, four, five months after your last shot, now is the time to go ahead and get it."

Safety and efficacy data look promising

These new boosters were tested on mice rather than people, a controversial strategy aimed at saving time (it's not unprecedented, however, as flu shots are changed each year without being routinely tested).

Looking at the data, Walensky says health authorities are confident about how well the vaccines will work and how safe they will be.

That data includes the 600 million doses of the original vaccine that have been administered across the country with what Walensky calls "an extraordinary safety record." Officials also saw similar safety results for an earlier version of this bivalent vaccine (meaning it targets two strains) that was tested in some 1,400 people.

That booster targeted the original coronavirus strain as well as the omicron BA.1 strain, as opposed to the more prevalent BA.4 and BA.5 subvariants targeted in the newly authorized version of the shot.

"So there are very subtle differences, but we have no reason to expect that this is going to have any different safety signal than either the 600 million doses we previously have given or these other bivalent boosts against omicron," Walensky says.

What's already clear, she adds, is that protection against the virus wanes over time, and that a booster will restore protection against infection, severe disease and death. She also points to lab studies that show this updated booster improves immune responses against other SARS-CoV-2 variants as well as similar responses to the original variant.

"So we have every reason to expect that it'll work just as well, and likely better," she says.

This interview was produced by Kaity Kline and edited by Simone Popperl.