Fully vaccinated people are exhaling this weekend, ditching masks and easing up on social distancing, per the latest Covid-19 guidance put out by the Centers for Disease Control and Prevention. The new advice marks a significant milestone in the nation’s effort to stamp out the disease, and signals the beginning of a return to normalcy. But scientists say there are good reasons not to toss out your mask stash just yet.

“It’s important to not see this change as a signal that this means that the pandemic is over or that there is no capacity for policy reversals in the future,” said William Hanage, an epidemiologist at Harvard’s T.H. Chan School of Public Health.

Given the incredible efficacy of the vaccines — in particular the Pfizer/BioNTech and Moderna vaccines — and the impact they’re having on bringing down Covid cases, it makes sense to give immunized people something back, he said. And in light of recent vaccination slowdowns, it could provide an extra incentive to anyone who is eligible but still holding off. But Hanage said the policy shift is as much about the changing weather as it is about vaccines.

“It’s a reflection of how much of a better place we’re in now than we were, but it’s a reflection too of the decreased transmission we expect to see over the summer months,” he said. And that means that people should prepare for Covid restrictions to be revisited in the fall, especially given uncertainties around how emerging variants will impact vaccines’ effectiveness in preventing disease spread. The Johnson & Johnson vaccine in particular, said Hanage, appears to be less effective at blocking transmission.

“There is a certain advantage to normalizing behaviors, like mask-wearing, that are going to be useful if we encounter any sort of bumps in the road in terms of variants or a serious seasonal effect,” he said.

Aerosol scientists like Linsey Marr, at Virginia Tech, point out that adopting a culture of occasional mask-wearing could have benefits that extend beyond the pandemic. “Not only does a mask help with COVID-19, but it also helps with other respiratory viruses, particulate air pollution, and pollen,” she told STAT via email. Colder, drier air brings with it the risks of other respiratory infections, including colds and the flu. With the world hunkering down last winter, most of those diseases declined drastically. Some disease modelers are predicting that could portend more severe flu seasons in the future. Masks would help mitigate those risks. (So would getting your flu shot.)

The CDC’s updated guidance advises vaccinated people that they can drop the masks in all outdoor and indoor settings, with the exception of hospitals, clinics, nursing homes, homeless shelters, prisons and jails, and planes, trains, and any other form of public transportation. But citizens are still subject to local masking requirements. Governors in about half of U.S. states had already lifted mask mandates. More are expected to expire at the end of this month.

Marr, for her part, was surprised the CDC’s move to unmasking came so soon. Not because the science isn’t solid, but because it’s such an abrupt shift. “I think it would have made more sense to give people and businesses advance warning,” she wrote. “So that those who have not had a chance to get vaccinated have time to get fully vaccinated before the change.” That includes the millions of 12- to 15-year-olds who only became eligible to receive Pfizer’s vaccine last week.

At least for now, children under the age of 12 don’t have access to any vaccine, so they will still be at risk of infection. So will millions of cancer patients, those with chronic illness, and other immunocompromised individuals — Covid-19 vaccines either aren’t recommended, or they just don’t work very well. Mask requirements made it easier for them to navigate public spaces with confidence that any contagious people they might encounter had at least a few layers of material strapped to their faces blocking the majority of infectious aerosols from flooding their shared air.

“For Covid, because it’s mostly respiratory transmission, the mask is what does the job when you’re closer to other people,” said Ann Petru, a pediatric infectious disease physician at UCSF Benioff Children’s Hospital in Oakland, Calif. Although kids tend to tolerate Covid-19 infections better than adults, the risk of bad outcomes is not zero. More than 300 have died, and roughly 15,000 have been hospitalized — far more than in a typical flu season. She worries that the honor system will allow unvaccinated adults to seize the opportunity to go mask-free, making indoor spaces where infectious aerosols can accumulate that much riskier for vulnerable individuals.

“A global statement for all of society is going to be overinterpreted by the wrong groups and then there will be more cases because people are not going to be protecting themselves as well,” said Petru. “So I don’t think it’s time to throw masks away in any kind of social scene where you’re indoors with others.”