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Vaccinated People Can Spread The Delta Variant, CDC Research Indicates


The CDC released new information today showing just how contagious the delta variant is. It explains why the agency updated its mask guidance this week, saying even vaccinated people should mask up indoors in public places. Joining us now to talk about all of this is NPR's Michaeleen Doucleff. Hey, Michaeleen.


CHANG: Hi. All right, so this report focuses on a big outbreak on Cape Cod during the Fourth of July holiday. Can you just tell us what happened there and why it's so significant?

DOUCLEFF: Yeah. So after the Fourth, the CDC became aware of an outbreak centered around several large public gatherings. So these were parties and events where people were densely packed inside and outside at bars, restaurants and rental houses. The CDC did some genetic sequencing of the virus circulating there, and about 90% of it was the delta variant. But they also did something else. They measured the amount of virus in some people's noses. And what they found - now, this is key - is that the people who were vaccinated were growing just as much virus in their noses as those who weren't vaccinated. So what this study shows is that people who are immunized can transmit the virus and possibly just as much as those who aren't immunized.

CHANG: And that is different from previous strains, right?

DOUCLEFF: Quite different. So with earlier variants, there were signs that vaccinated people were less likely to spread it because they had less virus in their nose and they carried around it for a much shorter period of time. But that does not seem to be the case with the delta strain. I talked to Jeremy Luban at the University of Massachusetts Medical School. He says this finding clearly changes what needs to be done to control this virus.

JEREMY LUBAN: It raises an alarm, which is basically that we're dealing with a new creature here, in delta, in terms of its transmissibility. So it's much more transmissible. So each person who is infected has the possibility of transmitting the virus to many more people.

DOUCLEFF: And the shift is really the reason the CDC has changed its mask guidance - telling vaccinated people they should also wear a mask indoors in communities where the virus is spreading. And if you really don't want to catch it, avoid large indoor public gatherings.

CHANG: Right. OK, and this comes on top of some internal CDC documents that were published by The Washington Post yesterday that show the delta variant may also cause more severe illness, right? Tell us more about that.

DOUCLEFF: Yeah, so this is kind of a preliminary finding, but it comes from three studies in Canada, Singapore and Scotland. And all these studies suggest that if you're infected with the delta variant, you may be about three or four times as likely to be hospitalized and twice as likely to die than if you were infected with previous variants. Now, the good news is these studies show that the vaccine being - the vaccines being used here in the U.S. really work well. They are still highly protective. For instance, three studies show the Pfizer vaccine provides more than 90% protection against hospitalization and deaths.

CHANG: OK, that is reassuring.


CHANG: Finally, we know that older people have been hit the hardest by this virus. Did these documents tell us anything more about them in particular?

DOUCLEFF: Yes. And this is a good reason to wear a mask inside, even if you're vaccinated, and avoid big indoor gatherings. Studies in nursing homes show that while the vaccine is still quite protective against hospitalization and deaths, it is not as protective in this older group or people who are immunocompromised. And one of the recommendations in the CDC report is to consider mandating vaccinations for all health care providers to protect those vulnerable populations.

CHANG: That is NPR's Michaeleen Doucleff. Thank you, Michaeleen.

DOUCLEFF: Thank you, Ailsa.


Michaeleen Doucleff, PhD, is a correspondent for NPR's Science Desk. For nearly a decade, she has been reporting for the radio and the web for NPR's global health outlet, Goats and Soda. Doucleff focuses on disease outbreaks, cross-cultural parenting, and women and children's health.