With aerosol transmission, safety generally increases with distance; the farther the aerosols travel, the more they diluted become. “It’s like being close to a smoker,” Dr. Marr said. “The closer you are, the more you’re going to breathe in.”

And distance aside, the more people there are in a room, the higher the odds that one of them will be infected with the coronavirus. A six-foot rule helps reduce that risk, said Donald Milton, an aerosol expert at the University of Maryland: “If people are six feet apart, you can’t pack them in. And so, it’s safer just because it’s less dense.”

Masks and good ventilation do a lot to reduce the risk. With these measures in place, the difference between three and six feet was likely to be relatively small, scientists said. And if Covid-19 is not very prevalent in the surrounding community, the absolute risk of contracting the virus in schools is likely to remain low, as long as these protections are in place.

“We can always do things to reduce our risks further,” Dr. Marr said. “But at some point, you reach diminishing returns, and you have to think about the costs of trying to achieve those additional risk reductions.”

Some experts say that a small increase in risk is outweighed by the benefits of fully reopening schools. “Trying to follow the six-foot guideline should not prevent us from getting kids back to school full time with masks, with at least three-foot distancing,” Dr. Marr said.

Others said it was too soon to loosen the C.D.C. guidelines. “Ultimately, I think there could be a place for this changing guidance,” Saskia Popescu, an infectious disease epidemiologist at George Mason University, said in an email. “But it’s not now, when we are struggling to vaccinate people, we’re still seeing over 60,000 cases a day and we’re trying to not reverse the progress we’ve made.”

Even proponents of changing the guideline say that any shift to looser distancing will have to be done carefully, and in combination with other precautionary measures. “If you’re in an area where there’s not a strong tendency to rely on masks, I don’t think it would be wise to extrapolate our data to that environment,” Dr. Perkins said.

Moreover, officials risk muddying the public health messaging if they establish different standards for schools than for other shared spaces. “I’ve evolved on this,” Dr. Linas said. “Last summer I felt like, ‘How are we going to explain to people that it’s six feet everywhere except for schools? That seems not consistent and problematic.’”

But schools are unique, he said. They are relatively controlled environments that can enforce certain safety measures, and they have unique benefits for society. “The benefits of school are different than the benefits of movie theaters or restaurants,” he said. “So I’d be willing to assume a little bit more risk just to keep them open.”

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