AILSA CHANG, HOST:

This afternoon a vaccine advisory committee to the CDC voted unanimously to recommend that booster shots be made available to everyone 18 years and older. CDC director Rochelle Walensky has now greenlighted the recommendation. Earlier in the day, the FDA expanded authorization for both the Moderna and Pfizer vaccines, authorizing boosters for all adults. NPR's Allison Aubrey joins us now.

Hey, Allison.

ALLISON AUBREY, BYLINE: Hi there - good to be here.

CHANG: Good to have you. OK, so this is a big change - right? - from a few months ago, when boosters were recommended only for older people and only those at high risk. Can you just walk us through how all of this unfolded?

AUBREY: Yes, this is a big change. I mean, a few months ago, when advisers to the FDA and CDC were asked to weigh in on boosters, there wasn't a lot of data, Ailsa, to show that they were needed. Now, all along, President Biden's closest COVID advisers have maintained that the U.S. should be proactive, that we should not wait until protection from the vaccine wanes, that we should anticipate it. So the messaging from the White House got a bit ahead of the science, but now things have changed. There is data to show that boosters can be beneficial. And it's clear that among older people who were among the first to be vaccinated, immunity has begun to wane and that a booster dose can, just as the name implies, boost protection.

CHANG: OK. Well, when it comes to all adults, not just older adults, what's the new evidence to support boosters for every one of them?

AUBREY: Well, some of the most recent real-world data comes from England. Back in September, the U.K. government introduced a booster program targeting people 50 and older. I spoke to Dr. Anthony Fauci about a new analysis from the U.K. that points to a significant increase in protection against symptomatic COVID infection from a booster dose. Here he is.

ANTHONY FAUCI: What they showed is that if you look at the third dose in people whose protection has drifted down to about 63%, you boost it back up to at least 94%, which is really quite impressive.

AUBREY: And what you want to see from a booster, he says. Now, in the U.S., the CDC has pointed to data from nursing homes and long-term care facilities across the country which shows that the rate of disease is markedly lower among people in these facilities who have received the booster.

CHANG: OK. So that's for slightly older people. But what about younger people in their 20s and 30s who are relatively healthy? Do they really need a booster?

AUBREY: You know, the stakes are definitely higher for older people. That's clear. There was a lot of discussion of that today, Ailsa, suggesting that the recommendation should be stronger for people 50 and up, that they should get the booster. But younger people do get breakthrough infections, and there's data showing a third shot does boost antibodies in young people and can be beneficial. Now, keep in mind right now COVID cases are rising. We're at about 88,000 cases a day. About 85% of counties in the U.S. have high or substantial levels of spread of the virus right now. So Dr. Fauci says given the situation and the season, boosters make sense.

FAUCI: So my recommendation would be, particularly as we go into the winter months and the holiday season where there's a lot of gathering indoors, I would recommend if you are eligible for a boost, go get boosted right now.

AUBREY: Because a booster can reduce your chances of getting infected, and if you don't get infected, clearly you can't spread it to anyone else.

CHANG: Absolutely. That is NPR's Allison Aubrey.

Thank you, Allison.

AUBREY: Thank you, Ailsa. Transcript provided by NPR, Copyright NPR.