People with breakthrough Delta-variant infections could transmit the virus to others.

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Just when you thought your days of masking up were mostly over, there's another COVID-related curveball. The US Centers for Disease Control and Prevention (CDC) on Tuesday updated its mask guidance, urging vaccinated people to wear masks if they're in certain areas of the country with significant levels of viral transmission.

As the Delta variant sweeps across the US, COVID-19 cases are on the rise again, particularly in certain hot spots. According to CDC data, more than 63% of US counties are in areas with "high" and "substantial" levels of community transmission.

In a media briefing on Tuesday, CDC Director Rochelle Walensky, MD, said the revised mask guidance is based on new scientific data showing that the Delta variant "behaves uniquely differently from past strains of the virus that causes COVID-19." The data indicate that, on those rare occasions where breakthrough infections occur, some vaccinated people "may be contagious and spread the virus to others," she explained.

"This new science is worrisome and unfortunately warrants an update to our recommendation," Dr. Walensky said.

Here's what the new mask guidance may mean for you.

What exactly is the CDC recommending, and who will be affected?

The CDC is urging vaccinated people to mask up when they're in indoor public settings in higher-transmission areas of the country. Those areas span wide swaths of the West, Midwest, and South.

cdc-transmition-rates
Credit: CDC

Areas of high and substantial transmission are designated in red and orange on the agency's COVID data tracker, seen above. Check the CDC site to find your own county. Or check in with your local public health department.

Dr. Walensky noted some areas are now reporting over 300 cases per 100,000 people over a seven-day period, "so really an extraordinary amount of viral transmission, which is what we're concerned about."

The indoor mask advice also applies to everyone in K-12 schools—teachers, staff, students, and visitors, regardless of vaccination status, she said. The recommendation is meant to ensure "that our kids can safely get back to full, in-person learning in the fall," the CDC director noted.

It's important to note that CDC already recommends masks for young children, ages 2 to 11, for whom a COVID-19 vaccine has not yet been approved.

Why is CDC reversing course on masks for vaccinated people?

While stressing that vaccination remains highly protective against severe illness, hospitalization, and death, Dr. Walensky said it's still possible for people who've already had their jabs to have breakthrough infections. With vaccine effectiveness rates of 90% to 95%, even fully vaccinated people are at risk of acquiring and transmitting COVID in areas where there is a high or substantial amount of transmission.

"We felt it was important for people to understand that they have the potential to transmit the virus to others," she told reporters. For example, a vaccinated person visiting an immunocompromised family member would want to take the necessary precautions not to pass along the disease.

Dr. Walensky acknowledged that people are tired and frustrated and that it's "not a welcome piece of news that masking is going to be a part of people's lives who have already been vaccinated." But she said the new data is based on emerging science that required a response. "This is not something that we took lightly."

The new advice represents a reversal of CDC's recent mask guidance. In May, the CDC said fully vaccinated individuals could go about their business without wearing a mask in most situations. Exceptions included local mask mandates, workplace and business requirements, and public transportation.

CDC's indoor masking recommendation comes at a moment when some local leaders are taking matters into their own hands due to a rise in Delta infections. New mask mandates recently have been announced in places like Los Angeles and St. Louis counties, among others.

What about unvaccinated people?

The agency's advice for unvaccinated people has remained unchanged: Keep masking up indoors. And get vaccinated.

"The vast majority of severe disease, hospitalization, and death is almost exclusively among unvaccinated people, which is why we still very much want to double down on making sure people continue to get vaccinated," Dr. Walensky said.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDCWHO, and their local public health department as resources.

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This story originally appeared on health.com