AILSA CHANG, HOST:

I'm Ailsa Chang in Los Angeles with today's developments on booster shots. Advisers to the FDA voted to recommend a booster dose for everyone who has received the Johnson & Johnson COVID vaccine. They say it could help improve immunity for the 15 million people who have received that shot. And here to tell us more is NPR health reporter Pien Huang.

Hi, Pien.

PIEN HUANG, BYLINE: Hey, Ailsa.

CHANG: All right. So tell us more about this recommendation by the FDA's advisers today.

HUANG: Yeah. So all 19 members of the FDA's Vaccine Advisory Committee voted to recommend a second shot of the J&J vaccine. And this is for anyone 18 and older who already got the vaccine, and it should come at least two months after getting that first shot. So a second shot was recommended because the J&J vaccine has been less effective than the Pfizer and Moderna vaccines, and getting a second dose could bring it up to a similar level.

Now, there are some open questions about whether the second shot should be considered a booster shot or just part of the primary course. Like, should the J&J vaccine just be a two-dose vaccine? There's also questions about the timing of that second shot. Like, maybe waiting until six months later could make for an even stronger immunity boost. But overall, the vaccine committee looked at the evidence and said people who got the Johnson & Johnson vaccine should get a second shot at least two months after so they're better protected against getting very sick from COVID.

CHANG: And what about the evidence that these advisers took a look at? Like, how did they come to this conclusion?

HUANG: Well, the decision ended up unanimous, but there were some real twists and turns in today's meeting to get there. It kicked off this morning with a presentation from Johnson & Johnson, and they were making the case that their vaccine was holding up well. Some studies, they said, show that immunity is lasting and it works against the delta variant. And then FDA official Peter Marks stepped in and said, that's not the whole story.

PETER MARKS: There are data that suggest the effectiveness of this vaccine is actually less robust than the company's presentation here.

HUANG: Amanda Cohn, a committee member from the CDC, says that their data shows that the J&J vaccine just hasn't been as protective as the Pfizer or Moderna vaccines, the so-called mRNA vaccines. In fact, in the U.S., the J&J vaccine has been just 68% successful at keeping people from getting hospitalized with COVID.

AMANDA COHN: Which is both lower than what we saw from that real-world effectiveness presentation, and it's also substantially lower than the mRNA vaccines.

HUANG: In comparison, Moderna has been more than 90% effective. The bottom line here is that the J&J vaccine was less effective than Pfizer or Moderna to start with, and there's still a gap. So the panel's recommendation now is that anyone 18 and older who got one dose of the J&J vaccine should go get another. Now, this is different from the recommendation they made yesterday for the Moderna booster. That one was limited to those 65 and older and others at high risk of getting COVID.

CHANG: Right.

HUANG: And that's partly because the first two doses seem to be holding up well in most people for the Moderna shot.

CHANG: I also understand that there was some talk at the meeting today about mixing and matching booster shots. Is that right?

HUANG: Yeah, that's right. There was a presentation of a government-funded study today that seems to show that following the J&J vaccine with an mRNA shot, particularly Moderna, could be better than getting another J&J shot and that it can sometimes produce high levels of antibodies. Now, that's not a formal recommendation at the moment. The caveat there is that the study is small and ongoing. Still, committee member Ofer Levy, a Harvard Medical School professor, says the results are important because it can help guide people to make more informed decisions.

OFER LEVY: Many Americans are taking matters into their own hands. And I'm reading in the media that people are getting boosters or mixing different products. And so in the real world, all these kind of combinations or extra boosters are already happening.

HUANG: The committee didn't vote on mix-and-match boosters today, but it seems like the FDA might leave some wiggle room for the CDC to make recommendations. So there's more to come next week, specifically next Thursday, when the CDC's Vaccine Advisory Committee takes this up.

CHANG: That is NPR's Pien Huang.

Thank you, Pien.

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