CDC on Friday released new physical distancing guidelines for schools, lowering the required distance from six feet to three feet, in response to new research showing three feet of separation was sufficient to stem the spread of the coronavirus in schools.

Why 6 feet?

According to the New York Times, the origins of CDC's original guidance recommending six feet of separation  are "something of a mystery." As the Times explains, experts thought when the virus first emerged that it was transmitted via large respiratory droplets—droplets that, according to significantly older studies, likely didn't travel more than three or six feet. That research, in conjunction with a necessarily cautious approach, may have led to CDC's recommendation calling for six feet of social distance, according to Linsey Marr, an expert in viral transmission at Virginia Tech University.

But the six-feet recommendation is not universal. For example, although the American Academy of Pediatrics recommends schools maintain three to six feet of separation, the World Health Organization recommends just one meter (about 3.3 feet) of separation.

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"There's no magic threshold," Benjamin Linas, a specialist in infectious diseases at Boston University, said. "There's risk at six feet, there's risk at three feet, there's risk at nine feet. There's risk always…The question is just how much of a risk? And what do you give up in exchange?"

According to the Times, experts over the past year have learned a lot more about the virus—in particular, that it travels through small airborne droplets, called aerosols, rather than through respiratory droplets. And aerosols, unlike respiratory droplets, can travel long distances and move in unpredictable ways through an indoor space, the Times reports.

That said, safety from aerosol transmission generally increases with distance, as aerosols become more diluted as they travel, the Times reports. "It's like being close to a smoker," Marr said. "The closer you are, the more you're going to breathe in."

New research suggests 3 feet of separation could be safe

However, a new study published in the journal Clinical Infectious Diseases found that just three feet of separation might be sufficient in schools, provided other safety measures are in place.

For the study, Westyn Branch-Elliman, an infectious disease specialist at the VA Boston Healthcare System, and colleagues assessed 242 school districts in Massachusetts between September 24, 2020, and January 27, 2021. The researchers selected schools in Massachusetts because the state advised schools to maintain between three and six feet of social distancing when they opened in fall 2020, meaning social distancing policies varied by school throughout the state.

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The researchers found that different social distancing policies resulted in no statistically different effect on Covid-19 rates, so long as other precautions—such as mask wearing and ventilation—were in place. In fact, the team found that Covid-19 rates were consistently lower in schools than they were in the surrounding communities, no matter whether the social distancing policy was three or six feet.

"[P]rovided we have universal masking mandates," Branch-Elliman said, "I think it's very reasonable to move to a three-foot recommendation."

However, the study authors acknowledged several caveats to their findings. For instance, they pointed out that even the schools that permitted a distance of three feet could have in practice maintained a larger degree of separation—so the findings could have captured just official policy rather than real-world implementation. The researchers also noted that the study did not look at whether new cases were transmitted in or outside the schools, and that many of the schools they assessed did not test for asymptomatic spread.

Separately, CDC on Friday also published a study looking at 20 elementary schools in Utah that did not have the space to provide six feet of separation between students.

"Despite high community incidence and an inability to space students' classroom seats more than six feet apar, this investigation found low [coronavirus] transmission and no school-related outbreaks in the 20 Salt Lake County elementary schools," the researchers wrote. They added that mask use at the schools was at 86%.

CDC updates guidelines

In light of this new research, CDC on Friday issued new guidelines recommending that, "with universal masking, students should maintain a distance of at least three feet in classroom settings." However, the recommendations still state that teachers and other staff should continue to maintain six feet of distance between each other and between themselves and students.

The recommendations apply to elementary schools, as well as middle and high school students unless those older students live in an area with high Covid-19 rates. In that case, if schools cannot keep students and teachers in assigned groups, CDC recommends students continue to maintain six feet of distance.

CDC also recommends that all students, regardless of community spread, maintain six feet of distance in areas or during times when masks cannot be worn, such as when eating lunch. The agency added that all students should wear masks when participating in certain activities—such as choir, band, or intense sports—where their breathing may deepen, adding that such activities should ideally take place outdoors or in a large and well-ventilated space. 

"CDC is committed to leading with science and updating our guidance as new evidence emerges," CDC Director Rochelle Walensky said in a statement. "Safe in-person instruction gives our kids access to critical social and mental health services that prepare them for the future, in addition to the education they need to succeed"  (Anthes, New York Times, 3/16; Rabin, New York Times, 3/14; Flaherty/Haslett, ABC News, 3/15; Wamsley, NPR, 3/15; Edwards, NBC News, 3/19; Kamenetz/Turner, NPR, 3/19; Lovelace Jr., CNBC, 3/19).