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In most parts of the US, the Centers for Disease Control and Prevention (CDC) now recommends masking indoors for everyone, including those who are fully vaccinated. Rochelle Walensky, the CDC director, made the announcement in a press conference on July 27. 

“This is not a decision that we or CDC have made lightly,” she said on Tuesday, adding, “We have mental health challenges in this country, we have a lot of continued sickness and death in this country. Our health systems are, in some places, being overrun from what is preventable.” 

The new agency guidance states that all people, regardless of vaccination status, should wear a mask in indoor public spaces, if located in regions of the country experiencing “substantial” or “high” COVID-19 transmission.

The CDC announcement also included updated guidance for K-12 schools, recommending that all teachers, students, staff, and school visitors wear masks while indoors, regardless of local transmission rates.

Here’s how to make sense of the announcement, and what it means for you. 

Why the masking change?

Rising case numbers, the surge of the hyper-contagious Delta variant, and new scientific data about transmission all play a role in the CDC’s decision. 

[Related: Will we need booster shots? And other questions about the Delta variant, answered.]

“I have said throughout my tenure at CDC that our guidance and recommendations will follow the science in our efforts to protect the health of as many Americans as possible,” said Walensky, adding that the shift was at the advice of many experts and in response to new science.

In May, the CDC made the controversial decision to revoke the indoor mask guidance for vaccinated people. So this new announcement is a reversal. But, reversing a previous decision isn’t the same as “flip-flopping,” says Celine Gounder, an epidemiologist and infectious disease specialist at New York University’s Langone Health in New York City. 

“The virus continues to mutate, continues to change, and it is outracing us in some ways,” she says, explaining that the agency has likely been carefully weighing the political cost of more restrictions against the weight of the emerging scientific data. Gounder emphasizes that the CDC is “trying to keep up with how the virus is changing. And, if anything, some of us think that the CDC should have changed its masking guidance even sooner.”

Personally, she says, she never stopped wearing her mask while in public indoor spaces. “Until I see vaccination rates across the city, across the five boroughs, really go up, until I see transmission rates across the five boroughs come down, I’ll likely continue to wear a mask indoors.”

At the time of the agency’s previous May announcement, case numbers were rapidly declining across the country and the more virulent Delta variant was less common. However, cases have been sharply increasing over the past few weeks and CDC data suggests the Delta variant is responsible for more than 80 percent of new cases. 

Walensky also cited currently unpublished data suggesting that vaccinated people infected with the Delta variant, in rare “breakthrough” cases, may carry levels of the virus comparable to unvaccinated individuals who contract it. 

This means that infected, vaccinated people who experience very mild or even no symptoms might still be able to transmit the virus to others at the same rate as those who are unvaccinated. And, because vaccinated people are likely to be asymptomatic, it can be difficult to know if you’re infected. 

For vaccinated people who catch the Delta variant, “there’s still so much virus in their nose, that it’s a good idea for them to wear a mask,” explains Anna Bershteyn, a professor of population health at New York University’s Grossman School of Medicine. She calls this new information “very surprising,” but adds that “there are a lot of vaccines out there that we all take, that prevent you from getting sick,” but don’t prevent transmission, citing the polio vaccine as one example.

However, in an email, Stacey Schultz-Cherry, an infectious disease researcher at St. Judes Children’s Research Hospital in Memphis, Tennessee, cautions that “we need to be careful in interpreting this data. It has not been published and is based on a small number of samples.” She further explains that understanding the method behind the results is important and that not all of the viral pieces detected in a standard COVID PCR test are contagious.

Nonetheless, there is a possibility that vaccinated people are contributing to the present, emerging US coronavirus wave, and the new recommendation seeks “to help prevent the spread of the Delta variant and protect others,” said Walensky, particularly those who can’t get vaccinated, like young children, or those for whom the vaccine is less effective, like the immunocompromised. 

What does it mean for you?

If you live in an area categorized as having “substantial” or “high” transmission rates by the CDC’s criteria, you should follow federal guidance and wear a mask in indoor public settings. Currently, Maryland and Vermont are the only states with no counties reaching the substantial transmission threshold. And, no matter where you are in the country, you should mask-up indoors in K-12 school settings.

But even if you live in an area with less transmission, and aren’t in a school you should continue to wear a mask indoors if you live with someone who is immunocompromised or unvaccinated, says Bershteyn.

The CDC’s transmission rate categories are based on two separate criteria: percentage of positive coronavirus tests and number of new cases per 100,000 people. These are “very standard ways of categorizing what are high risk places,” according to Gounder. Both metrics are considered over a seven-day period. Substantial transmission is defined as 50-99.99 average new cases per 100,000 people in the past seven days, or as 8-9.99 percent of tests returning positive over a week. Meeting either threshold is enough to categorize a county’s transmission as substantial thus fall under the new guidance. Anything above the substantial range is classified as high transmission, which also means everyone venturing out in public should wear masks indoors.

People should look to the CDC guidance, but also to their local officials, says Gounder. “The CDC sets guidance for the country. Ideally state and local health departments should be tailoring that guidance based on the epidemiology of [their] community.” 

Bershteyn agrees. “We’re a big country with a lot of different things going on,” she says. “You have to do what your local authorities say to do as well.”

[Related: Despite surging Delta variant, a national vaccine mandate is unlikely]

Most importantly, every source that spoke with PopSci stressed that all three available vaccines, particularly the Pfizer and Moderna mRNA vaccines, remain extremely effective at preventing COVID-19 symptoms, severe disease, hospitalization, and deathーeven with Delta variant. Getting vaccinated is still the number one thing you can do to protect yourself and others. 

How to keep track of your local situation

The CDC website updates its national map of transmission rates daily. Check here to find out how your county’s transmission level ranks.

All US states and territories also maintain health department websites. You can find out your state’s current COVID-19 policies and guidelines by visiting its health department website and clicking on the COVID-19 section.

Other helpful resources include this handy, regularly updated list of coronavirus-related restrictions in every state, published by the American Association of Retired Persons (AARP), and the COVID-19 Policy Tracker dashboard, which contains information from both state and local agencies. You can also build your own coronavirus dashboard to monitor case trends in areas important to you through The New York Times’ website.

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