The Centers for Disease Control and Prevention is clear and consistent in its social distancing recommendation: To reduce the risk of contracting the coronavirus, people should remain at least six feet away from others who are not in their households. The guideline holds whether you are eating in a restaurant, lifting weights at a gym or learning long division in a fourth-grade classroom.

The guideline has been especially consequential for schools, many of which have not fully reopened because they do not have enough space to keep students six feet apart.

Now, spurred by a better understanding of how the virus spreads and a growing concern about the harms of keeping children out of school, some public health experts are calling on the agency to reduce the recommended distance in schools from six feet to three.

“It never struck me that six feet was particularly sensical in the context of mitigation,” said Dr. Ashish Jha, dean of the Brown University School of Public Health. “I wish the C.D.C. would just come out and say this is not a major issue.”

reviewing the matter.

The idea remains contentious, in part because few studies have directly compared different distancing strategies. But the issue also boils down to a devilishly difficult and often personal question: How safe is safe enough?

“There’s no magic threshold for any distance,” said Dr. Benjamin Linas, a specialist in infectious diseases at Boston University. “There’s risk at six feet, there’s risk at three feet, there’s risk at nine feet. There’s risk always.” He added, “The question is just how much of a risk? And what do you give up in exchange?”

The origins of six feet

relatively low-risk environments; children under 10 seem to transmit the virus less readily than adults.

In recent months, there have been hints that six feet of distancing may not be necessary in school settings. Case rates have generally been low even in schools with looser distancing policies. “We know lots of schools have opened up to less than six feet and have not seen big outbreaks,” said Dr. Jha.

In a 2020 analysis of observational studies in a variety of settings, researchers found that physical distancing of at least one meter substantially reduced transmission rates of several different coronaviruses, including the one that causes Covid-19. But they found some evidence to suggest that a two meter guideline “might be more effective.”

“One of the really important data points that has been missing is a direct head-to-head comparison of places that had implemented three feet of distance versus six feet of distance,” said Dr. Elissa Perkins, the director of emergency medicine infectious disease management at Boston University School of Medicine.

the team reported in the journal Clinical Infectious Diseases last week. The study also found that Covid-19 rates were lower in schools than in the surrounding communities.

The authors say the findings provide reassurance that schools can loosen their distancing requirements and still be safe, provided they take other precautions, like enforcing universal mask wearing.

“Masking still appears to be effective,” said lead investigator Dr. Westyn Branch-Elliman, an infectious diseases specialist at the VA Boston Healthcare System. “And so, provided we have universal masking mandates, I think it’s very reasonable to move to a three-foot recommendation.”

Not everyone finds the study so convincing. A. Marm Kilpatrick, an infectious disease researcher at the University of California, Santa Cruz, said that the school-district data was too noisy to draw firm conclusions from. “It doesn’t really allow you to get, I think, an answer that you can feel really confident in,” he said.

The study’s authors acknowledged that they could not rule out the possibility that increased distancing provided a small benefit.