The Centers for Disease Control has officially recommended that people living in areas with significant COVID transmission should start wearing masks again, regardless of vaccination status, as the Delta variant blows through the country. School-age kids should also be masking up again, said Rochelle Walensky, the director of the CDC, in a press call on Tuesday afternoon.

Nearly two-thirds of all US counties currently have significant or high levels of COVID transmission and at least one in every five people who are getting COVID are infected with the Delta variant.

This version of the coronavirus is more transmissible than previous variants, and though most cases have been seen in unvaccinated people, the new virus is causing breakthrough cases in some vaccinated individuals. There were 65,000 breakthrough cases (which are people who are fully vaccinated but got COVID-19), as of July 22. That’s 0.04 percent of all fully vaccinated folks, which include about 160 million people. Walensky noted that although most disease transmission is still occurring in unvaccinated people, even fully vaccinated people can still catch and transmit the virus to others. This is especially important for those visiting immunocompromised people or those who are unable to get vaccinated due to health conditions. In some cases, the CDC is finding that viral loads are similar in breakthrough infections as in infections in unvaccinated people, according to Walensky.

The mask advisory is also in part to try and prevent the coronavirus from accumulating other mutations that would enable it to evade our vaccines, said Walensky. More infected people means more opportunities for SARS-CoV-2 to divide and mutate.

Walensky also announced that the CDC is following cohorts of essential workers and health care professionals to gather more data on breakthrough cases.

Efficacy against the Delta variant varies by vaccine, but most still protect against severe COVID symptoms. That means even if you get sick, the vaccine will decrease the likelihood that you get seriously ill or die from COVID. In one recent study in the New England Journal of Medicine, for instance, two doses of the Pfizer vaccine were 88 percent effective at preventing symptomatic disease. It’s also 96 percent effective against hospitalization. Moderna has also reported that its vaccine is still very effective against the Delta variant, though haven’t published their data yet.

Vaccinated individuals are therefore still mostly protected against the Delta variant—the new masking guidelines are not a commentary on the efficacy of our vaccines, they’re in light of the high transmissibility of this new variant. We’re still in the process of learning about the Delta variant and are trying to prevent as many new COVID cases as possible. Wearing a mask regularly will further decrease everyone’s risk, both by helping to prevent the virus from escaping your nose and mouth if you have COVID and by keeping those same particles out if you’re uninfected.

It was just two months ago that the CDC announced vaccinated individuals would no longer need to wear masks except in certain high-risk scenarios, but this is not necessarily a reversal of that decision. The earlier decision was made based on older data that largely didn’t reflect how transmissible the Delta variant is. Now that we have better data, the CDC is reevaluating.

The country is currently facing a massive upswing in COVID cases—we are approaching the levels seen this time last year, when many states were locked down. To know what the transmission levels are at in your area, you can check the CDC county COVID tracker here.

The major difference now, however, is that roughly half the country is now fully vaccinated. That means a surge in cases will hopefully not be as catastrophic as last time, since the vaccines should help decrease the disease severity. Ideally we would prevent COVID transmission entirely, but the goal of vaccination is mainly to prevent symptomatic disease, especially the severe variety, and to prevent deaths. Our current vaccines do that extremely well, even against the Delta variant.

Masking, however, is once again important from a public health perspective. Every little bit we do prevents some transmission, which adds up to fewer COVID cases and fewer deaths. Walensky emphasized on Tuesday the importance of getting vaccinated, now more than ever. “This moment, and most importantly, the associated illness, suffering, and death could have been avoided with higher vaccination rate.”

Correction July 29, 2021: This story has been updated to reflect the fact that breakthrough cases as of July 22 accounted for 0.04 percent of all fully vaccinated people, which includes 160 million people.