AILSA CHANG, HOST:

And here in the U.S., we're in another unsettling moment during this pandemic. Even people who are vaccinated are wondering if it's time to be cautious again, given the rise in cases, the fast-spreading delta variant and reports of vaccinated people testing positive - from Olympic athletes to Texas lawmakers to a White House staffer. NPR health correspondent Allison Aubrey is here with some guidance.

Hi, Allison.

ALLISON AUBREY, BYLINE: Hi there, Ailsa. Good to be here.

CHANG: Good to have you. OK. So people who are fully vaccinated, like myself, are looking for some reassurance right now that they are still going to be protected against this delta variant. Can you offer any reassurance?

AUBREY: Yeah. The vaccines are incredibly effective against serious illness from the delta variant. For example, a recent study from the U.K. found that the Pfizer vaccine is 96% effective against hospitalization from the delta variant after two doses.

Now, I talked to Dr. Marc Boom. He's the CEO of Houston Methodist Hospital. He says 90% of the people hospitalized there now with COVID are unvaccinated, and they have doubled the number of COVID patients they had two weeks ago.

MARC BOOM: Every indicator right now is flashing bright-red at us and moving at a very quick rate. We're very, obviously, disappointed in this. And, frankly, this was entirely avoidable if everybody had gone and gotten vaccinated, as we've all been urging.

AUBREY: So, clearly, unvaccinated people remain at risk.

CHANG: Right. But what about fully vaccinated people testing positive right now? Like, should we be freaking out?

AUBREY: You know, there is a very big difference between simply testing positive and getting sick enough to end up in the hospital with COVID. The whole promise of the vaccines is that they would protect against serious illness and death, and they do. It was not realistic to think that they'd eradicate the virus entirely, so it is not a surprise to hear about a positive test, or even a mild case, in a fully vaccinated person.

I think what is concerning is that the virus is circulating widely again due to this super-transmissible delta variant, and there are some breakthrough cases that have led to serious illness. The CDC has reported about 5,500 patients with breakthrough infections who've been hospitalized or died. That's out of 160 million people fully vaccinated in the U.S. But in most cases, Dr. Boom tells me, these patients are vulnerable.

BOOM: By and large, those individuals have underlying significant health risks, like cancer, like transplant, that probably prevented them from mounting a full immune response to the vaccine.

CHANG: Oh, interesting.

AUBREY: The CDC isn't tracking or sort of keeping count of mild breakthrough cases, so there's a bit of an information gap there. But life-threatening breakthrough cases are very rare.

CHANG: OK. So even though the really serious breakthrough cases are very rare, do I need to change my behavior? As someone who's fully vaccinated, do I need to change the way I'm going about life right now? Do I go back to masking, for example?

AUBREY: Well, with the delta variant, scientists say people who are infected seem to release more virus into the air. That's the way the virus spreads. And even if you are vaccinated, there's still a chance you can be exposed and get infected or maybe even pass it on. Los Angeles County, as you know, has already reinstated a mask mandate.

CHANG: Yep.

AUBREY: And many experts say it just makes good sense to mask up right now. I spoke to Dr. Bill Miller of the Ohio State University. He says, when you walk into a grocery store and you see all the unmasked people, it's concerning.

BILL MILLER: It doesn't make sense to see 95% of people not wearing a mask when we're nowhere near that target level in terms of vaccination. So I, personally, continue to wear a mask.

AUBREY: And this is especially true if you have kids or immune-compromised (ph) people in your household who may not be fully protected.

CHANG: Right. But do most infectious disease experts agree with Miller?

AUBREY: You know, every infectious disease expert I talked to says this is the time to be cautious. You don't have to cancel vacation plans or lock down, but masking is a simple and effective step. In fact, the American Academy of Pediatrics is recommending all kids 2 and up and staff be masked when school starts again, vaccinated or unvaccinated.

CHANG: That is NPR's Allison Aubrey.

Thank you, Allison.

AUBREY: Thank you. Good to be here. Transcript provided by NPR, Copyright NPR.