Updated July 20, 2021 at 3:02 PM ET

With the highly contagious delta variant fueling a rise in COVID-19 cases, Dr. Jerome Adams — the surgeon general under former President Donald Trump who once advised against mask-wearing — now says even the vaccinated may need to mask up.

Adams is part of a growing chorus of criticism of the CDC guidance, issued in May, that people who are fully vaccinated can resume activities indoors and outdoors without masks. The messaging, Adams told NPR's Morning Edition, "has just absolutely, unequivocally failed."

"More people than ever — vaccinated and unvaccinated — are going maskless," he said. "It doesn't seem to have convinced anyone to get vaccinated."

Adams said the CDC needs to change guidance on masking requirements again in light of new virus outbreaks and the more contagious delta variant.

The delta variant — which is more than 200% more transmissible than the original coronavirus — is pushing cases up in the United States: The number of people getting COVID-19 has nearly tripled since a month ago. Outbreaks are clustered in areas that have low vaccination rates.

But the spread is also happening among the vaccinated, and as a result, Adams said, new mask guidance may be necessary. Recently, new virus spikes led Los Angeles County to restore its indoor mask mandate, even for those who are vaccinated. And the American Academy of Pediatrics issued new guidelines for schools, recommending that everyone over age 2 – vaccinated or not — wear masks in classrooms this fall.

"We need both vaccinated and unvaccinated adults to take this seriously. And taking it seriously means if your local health officials say cases are going up and positivity rates are going up in your location, you may need to 'vax it and mask it,' " he said.

Adams said the problem with the CDC's current messaging is that what people heard was "no more masks." Plus, he said, the guidance was rolled out quickly and left many unanswered questions.

Businesses "just throw their hands up in the air and say 'Hey, we don't have a way to figure out who's vaccinated or who's not, so no masks for anybody,' " he said.

Here are excerpts of the Morning Edition conversation:

Does the guidance need to change?

I absolutely think that the CDC needs to come out and say to people, "Look, this virus is changing rapidly." Because here is something else that people don't understand. When you look at [data released by the Israeli government], the Pfizer vaccine was still incredibly effective at preventing severe disease and hospitalization but 64% effective at stopping spread. So even the vaccinated now could be spreading – much less spread relative to being unvaccinated but that's being counteracted by delta being so much more contagious. We cannot afford to send the message that if you're vaccinated you have no worries and no responsibilities.

How likely is it that vaccinated people will be able to catch COVID-19 and show symptoms?

Well, we still don't know that yet. And that's one of the challenges. ... The messaging that Tony Fauci and I had last year on masking – one of the things is this virus has humbled us. What we thought we knew changes very quickly. We thought there was low asymptomatic spread. We, quite frankly, were not suspicious enough of reports that there were high degrees of asymptomatic spread, and our messaging didn't reflect what ultimately came to be true. This delta variant is changing what we thought we knew. And again, when you look at boosters, particularly for people who are vulnerable or older, it may be that at least in terms of stopping spread, that we need to, again, focus on vaccinated as well as unvaccinated.

We don't know what we don't know, but that's frustrating and a little scary to know that you and Dr. Fauci didn't know as much or more than us last year. We're looking to you to tell us what to do and lead us.

Exactly. One of the things that we need to have comfort in is letting the public understand that we are trying to build the plane as we're flying it, that this virus is changing in real time, and that's how the scientific process works. Science isn't a destination. It's a journey. We need to have the courage to say, look, that we had this as our hypothesis, it didn't prove to be true, now we've got to course correct. The hypothesis that if we told people "vax it or mask it" it would encourage more people to get vaccinated has failed. So we need to change the message.

What should people be doing when they're considering going out, say to the store?

They need to consider where they're going, who they're going to be around, whether they're vaccinated or not and whether they can ensure themselves that the people around them are going to be vaccinated.

So me personally, if I'm going out around people who I all know are vaccinated, right now I don't wear a mask. If I go out around people who I'm not sure whether or not they're vaccinated, I'm going to be much more likely to wear a mask. And I'm always going to make sure I'm careful when I'm bringing my 11-year-old daughter around. Because something else that people need to remember is that when you talk about unvaccinated – a "pandemic of unvaccinated" – that includes our kids. That includes people with access issues, that includes older and vulnerable people. So you may need to "mask it and vax it" in the future.

Taylor Haney and Jan Johnson produced and edited the audio story. Heidi Glenn produced for the web. contributed to this story

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