Coronavirus cases and deaths in the U.S. are down dramatically from last winter's peaks, but the road ahead could still be a long one, with the rapid spread of the delta variant — now the dominant strain of the virus in the U.S. — and mounting questions over how effective current vaccines are against it.

Addressing those concerns in an interview Thursday with NPR's All Things Considered, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Disease, said studies continue to show that vaccines are not only effective against the virus, they're also highly effective at preventing serious disease or hospitalization.

"No matter what study you look at, the protection against severe disease leading to hospitalization is always well within the 90%, regardless of the study, regardless of the country," Fauci said.

That's just part of the reason why, he said, it's so crucially important to get vaccinated.

"It's so easy to get vaccinated. Viruses don't mutate if they can't replicate, and you can prevent them from replicating by vaccinating enough people so that the virus has nowhere to go," said Fauci, who is also chief medical adviser to President Biden.

Below are excerpts of the conversation, edited in parts for clarity and length:

Interview Highlights

New data out of Israel suggests the protection provided by the Pfizer vaccine may be waning, that its efficacy at preventing infection or symptomatic disease has dropped to 64%. A key caveat: They do still think the shot is highly effective at preventing serious disease or preventing death. But what is your read on this and these questions? Do the vaccines still work against this new variant?

The answer is yes, it does. If you look at the vaccines that we've been using here, and multiple studies from multiple countries show a high degree of efficacy, as you mentioned correctly, especially against severe disease leading to hospitalization. If you look at the effect against just infection itself or mildly symptomatic infection, the levels that we are getting in other studies seem to be substantially higher than the Israeli level of 64%. So what we really need to do before we can really make any determination, is to get a bit more information from our Israeli colleagues, which we're trying to do.

So are you skeptical of the data out of Israel?

No, no, I have a great deal of confidence in them. But before we try to extrapolate for the situation here, I would want to see a little bit more details of the data.

I saw some of the news out of the White House COVID-19 briefing today that here in the U.S., 99.5% of deaths from COVID-19 are in unvaccinated people. It does suggest strongly that the vaccinations are working at preventing death.

Yes, very much so. The data are so clear. And if you look in our own country, where the level of vaccination is low, the level of infection is increasing. And with that, you'll have hospitalizations and hopefully not but likely you would see increase in deaths — an overwhelming reason why we've got to get as many people vaccinated as we possibly can.

Are we also seeing more evidence that you really need to get both doses — if what you're getting is a two-dose vaccine like Pfizer or Moderna?

Overwhelmingly, yes, we are seeing that in any study you look at, the level of protection when you follow one dose versus the level of protection following two doses, is dramatically lower. People who feel, "Well, I have had one dose, do I really need a second dose?" — if you have a two-dose regimen, it is absolutely essential that you get your second dose.

Los Angeles recently began encouraging even vaccinated people to mask indoors. The California state Capitol just laid out a mask mandate for lawmakers and staff because four fully vaccinated people there tested positive for the virus. Where are we on this? Should local officials in other parts of the U.S. be thinking about, OK, maybe we need to go back to mandating masks?

Well, from the standpoint of, let's take the CDC recommendations that if you are fully vaccinated, you have a high degree of protection. So the recommendation remains that you don't need a mask whether you're indoors or outdoors if you are fully vaccinated.

The situation becomes a bit more complicated when you're in an area where you have two things: a low level of vaccination in the community and a high level of viral dynamics. Under those circumstances, you could understand how some local authorities might say, you know, you might want to make sure you get the extra added benefit of having a mask. Although that is not a CDC recommendation, it is something that some people are considering, particularly the elderly and those who might have an underlying condition — where they're not absolutely sure that they have an optimal immune response.

It's tricky because it's easier just to say yes or no, you need it or you don't. This is more nuanced.

Exactly, that's the point. There's a lot of variability among people in their own response and where they are and what their exposure is.

How worried are you that the Delta variant could mutate into something more aggressive, more worrisome, particularly with so many unvaccinated people still out and about?

Well, that is a concern, and that's the reason why we keep pushing, saying, please, people, if you're not vaccinated, seriously, consider it. It's so easy to get vaccinated. Viruses don't mutate if they can't replicate, and you can prevent them from replicating by vaccinating enough people so that the virus has nowhere to go.

If you give the virus free reign to circulate in the community, sooner or later it's going to mutate. And one of those mutations may be a mutation that makes it a more dangerous virus.

And in this case, like with the Delta variant, that's now the dominant one here, we've established it's more contagious. Do we know it actually causes more severe illness if you do catch it?

The data on the degree of transmissibility is very clear. There's no doubt it's more transmissible. It is likely when you get more data, that it is likely that it also can give you more serious disease.

Last thing, and this is back to masking, because so many people look to you as the guide to how to navigate this. Are you wearing a mask anymore in any circumstances?

You know, personally, I don't for the simple reason that my environment is so restricted. I spend very little time with people who are not vaccinated, so it isn't as if I'm out and about out there intermingling in places where there's a lot of infection. So for that reason, it's very unusual for me to wear a mask.

If you were headed to a crowded concert indoors in a state where the case rates are still high, would you think about it?

Being an 80-year-old person, I would seriously think about that. Yes.

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