The summer surge in COVID-19 cases is an unwelcome surprise for health officials and experts who thought, for a brief period, that the U.S. had the coronavirus pandemic largely under control.

"This is the low season. It shouldn't be spreading as fast during summer," says Ali Mokdad, who tracks coronavirus trends at the Institute for Health Metrics and Evaluation (IHME) at the University of Washington. Respiratory diseases such as the flu and COVID-19 usually spike in the winter, when people spend much of their time sequestered indoors.

But now, in the blazing heat of summer, hospitals in some parts of the country are again getting slammed with COVID-19 patients. Holiday gatherings over the Fourth of July — which President Biden once hoped would mark the country's independence from the virus — may have fueled new outbreaks. Cases, hospitalizations and deaths are ticking up after a long decline.

Back in mid-May, when over a million new people were getting vaccinated each day, the Centers for Disease Control and Prevention issued guidance declaring that it was safe — for fully vaccinated people — to shed their masks in most settings. "You can do things you stopped doing because of the pandemic," the CDC's director, Dr. Rochelle Walensky, said at the time.

Now, with vaccination progress stalling, a growing chorus of doctors and experts is saying the CDC let up on masking too early. They want the nation's public health agency to call for masking and mitigation measures in areas that are suffering, to stop the virus's spread.

"We need to reduce the circulation of the virus right now, so when we start the surge in the winter, we will be in a better position," Mokdad says.

Sound on science, poor on policy

The CDC's May guidance was based on sound science but led to poor policies, says Dr. Ali Khan, a former CDC official who's now at the University of Nebraska Medical Center's College of Public Health. The vaccines do protect people from getting seriously ill from COVID-19, so letting vaccinated people stop wearing masks made sense.

"The science was strong at that time," Khan says. The CDC intended for the announcement to encourage people to get vaccinated. It did for some, says Khan, but it also led to the de facto lifting of mask policies for unvaccinated people — just before the more transmissible delta variant started to take hold.

The summer surge, he says, is the result: "Those who didn't want to wear a mask and didn't want to get vaccinated are out there now spreading disease to others who are not vaccinated."

The timing of the guidance was off, says Jessica Malaty Rivera, an infectious disease epidemiologist who has worked with the COVID Tracking Project.

"When the Biden administration and the CDC [said] if you're vaccinated, you can take your mask off, I screamed," she says. "Because for so long, we've been talking about getting to a place where transmission is low and vaccination is high. Transmission was lower [at the time]. Vaccination was not high. So it seemed truly premature."

The tone of the messaging also signaled to vaccinated people that they could throw caution to the wind, Rivera says. "It gave people, in their minds, immunity passports," she says. "It said to them, 'I'm vaccinated — I can go to Miami. I'm vaccinated — I can go to this party in a basement.' "

The messaging around lifting mask use should have been more gradual, Rivera says: "You can change some of your behavior, but don't just have a rager as if the pandemic wasn't happening. Calibrating, instead of eliminating [all caution], was where we needed to go."

Calls for more masking, especially indoors

It's not too late for the CDC to change its tune now, particularly with a more transmissible virus spreading rapidly, says Dr. Carlos del Rio, an epidemiologist at Emory University: "In my mind, it would be advisable for CDC to say, given the spread of the delta variant in our country, we would recommend that people go back to using masks indoors," particularly in areas with huge spikes in cases.

"It would be helpful for the CDC to say, if your numbers get to a certain level, you should recommend masking," he says.

Implementing indoor mask mandates for everyone, regardless of vaccination status, is "the right and scientific thing to do," says IHME's Mokdad. "That's the only way we can send a signal to the public in the United States that we're not out of danger, [while] we encourage [more] people to go and get the vaccine."

Several health care organizations have recently called for universal masking for both vaccinated and unvaccinated people, including National Nurses United, a large nurses union, and the American Academy of Pediatrics, which recently recommended universal masking in schools. Rivera says this policy would be safer and easier to implement than the CDC's guidance, which offers different options for vaccinated and unvaccinated people.

"Who is going to be monitoring who is vaccinated and who is unvaccinated? Teachers, administrators, parents?" Rivera asks. She adds that having different rules for vaccinated and unvaccinated people "creates all kinds of issues of equity and judgment and access."

So far, the CDC stands by its statement that vaccinated people do not generally need to wear masks. "At this time, we have no intention of changing our masking guidance," wrote CDC spokesperson Jason McDonald, in bold text, in an email to NPR.

In a White House press briefing on July 22, the CDC's Walensky described mask-wearing, for vaccinated people, as "a very individual choice."

"You have exceptional levels of protection from that vaccine, and you may choose to add an extra layer of protection by putting on a mask," she said, while also suggesting that local officials in places with large gaps in vaccination coverage and rising case rates consider "additional measures."

The public is lulled by the fact that the summer surge is largely hitting unvaccinated young people, who are more likely to survive COVID-19, says Khan. But there's a danger that the surge in cases could spill back into more vulnerable groups, such as older adults and immunocompromised people.

"There are still many people in those high-risk groups that have not yet gotten the vaccine, and it's not 100% effective," he notes. Meanwhile, cases are rising rapidly week over week.

Vaccinated people are still protected. But the more virus that's circulating in the community, the more likely it is that people — both unvaccinated and vaccinated — will get it.

"What you don't want is disease to spread from younger people back into high-risk individuals, which would be a disaster," Khan says.

NPR's Allison Aubrey, Selena Simmons-Duffin and Rob Stein contributed to this report.

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