© CDC Covid Data Tracker People in orange and red-colored counties should wear masks indoors in public, the CDC said Tuesday. CDC Covid Data Tracker

• The CDC is now recommending that fully vaccinated people wear masks when indoors in public in certain areas.
• A federal, color-coded map shows the red and orange zones where masks are recommended.
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The Centers for Disease Control and Prevention is now recommending that fully vaccinated people put their masks back on when indoors in public, at least in the areas of the country where COVID-19 is spreading fastest.

CDC Director Rochelle Walensky said Tuesday that "the Delta variant behaves uniquely differently" compared to other versions of the virus, and that vaccinated people "may be contagious and spread the virus to others."

But the CDC isn't recommending that everyone mask up again.

The agency's new guidance is limited to places where COVID-19 transmission is deemed "substantial" or "high," meaning that there are either more than 50 cases per 100,000 people in the area, over a seven-day period, or that the COVID-19 test positivity rate is higher than 5%.

(The one glaring exception to that rule is in K-12 schools nationwide, where the CDC is now recommending everyone mask up to protect kids and teachers.)

The main reason for the change in the CDC guidance is that, in areas where there is a lot of virus circulating, the risk of getting infected, even for vaccinated people, is now very high. And while vaccination helps protect people from heading to the hospital, or eventually, dying from the disease, it is not a perfect shield against COVID-19. Vaccinated people can get sick, and prolong the pandemic too, by spreading their virus around.

"That's why we are saying, in areas of substantial or high transmission, even if you are vaccinated, that we believe it's important to wear a mask in those settings," Walensky added.

"There are some people who are not able to be fully vaccinated, like children, and some people who are not able to be fully protected even though they are vaccinated, like immunocompromised people. So part of the reason for this guidance is to make sure that we can protect those [people]."

Mask guidelines, broken down by county

To find out if you're in a place where transmission is substantial or high, you can use the CDC's COVID Data Tracker, which drills down to the county level.

Much of the US is currently seeing "substantial" (orange), or "high" (red) rates of transmission, where masks are now advised indoors. In the few pockets of "moderate" (yellow) and "low" (blue) zones, fully vaccinated people can go mask-free, according to the CDC.

As an example, here's what it looks like if you search "Dallas County, Texas" on the CDC's map:

© CDC Covid Data Tracker Transmission is high enough in Dallas County to warrant masks indoors in public, according to the CDC's new guidance. CDC Covid Data Tracker

Transmission is high in Dallas County, meaning people should wear masks indoors in public, according to the CDC.

Only three US states are entirely in red - Florida, Arkansas, and Louisiana - but Missouri and Mississippi are almost there, with only a few scarce orange, yellow, and blue counties.

Even in New York City, where more than 65% of adults are now fully vaccinated, transmission is in the red or orange zone in all five boroughs, meaning all New Yorkers should mask up indoors in public.

"It is not a welcome piece of news that masking is going to be a part of people's lives who have already been vaccinated," Walensky said, stressing that medical experts, when shown the data on Delta infections in the US, "have universally said that this required action."

This "could have been avoided with higher vaccination coverage," she added.