CDC: Color-coded map shows where Americans need to wear masks again, and where you can go maskless, outside of schools

• The CDC now recommends that fully vaccinated people wear masks indoors in public in certain areas.

• A color-coded map of the US shows the 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 US where COVID-19 is spreading fastest.

CDC Director Rochelle Walensky said Tuesday that "the Delta variant behaves uniquely differently" compared with 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 there are either more than 50 cases per 100,000 people in the area, over a seven-day period, or 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 a lot of virus is 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 the virus.

"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]."