As the Delta variant surged through the US this summer, the Centers for Disease Control and Prevention adjusted its definition of a close contact for students in classrooms—that is, for who would be contacted and urged to quarantine if they’d been near someone who later tested positive for COVID-19. Last school year, any child within 6 feet of an infected person for a total of 15 minutes or more over a 24-hour period needed to quarantine. This school year, with little fanfare, the CDC reduced this distance to 3 feet in classrooms, as long as both students consistently wore well-fitting masks. (If they didn’t, or if either person was an adult, or if the contact happened anywhere in the school other than the classroom, the 6-foot rule still applies.) 

This may not seem like a major shift, but in practice, it’s likely to mean that most kids who spent one or more school days in the same room as an infected classmate won’t be asked to quarantine or test. As New York City’s public school system shifted its policies last month to align with the new CDC guidelines, Annie Tan, a teacher in Brooklyn, told The New York Times that she thinks the change could lead to a rise in cases: “In terms of routine, sure, it’s more stable, but that doesn’t mean it’s safe.” 

“Translating the science into public policy can be difficult,” says William Schaffner, an infectious disease researcher at Vanderbilt University Medical Center. “How do you apply a quarantine rule? I think what the CDC is trying to do is provide a yardstick that will be reasonably easy to apply and doesn’t shut down the whole school.” 

Other considerations

Health risks aren’t the only consideration when adjusting distancing guidelines. The 3-foot rule for quarantining may also contribute to a more equitable educational system, forcing fewer kids to go home when they’re a close contact. Sara Bode of Nationwide Children’s Hospital says she’s had patients out of school on quarantine multiple times already this school year. “That is not an effective learning environment. They’re not learning,” she says. Kids are more likely to miss school when their families lack access to transportation and thus can’t get them tested to shorten their quarantine, she notes. “These kids already lost more school days last year than kids from higher socioeconomic classes, and we are just repeating that this year.”

Evidence showing that either the 3-foot close contact circle or the 6-foot cut-off that preceded it is capable of capturing the majority of secondary classroom cases (that is, cases transmitted within schools) is indirect. In its Science Brief, updated July 9, the CDC pointed to several studies and reports conducted over the last school year that showed transmission within schools happens, but is not explosive as long as everyone in school is masked and several other protective layers are in place. In the reports on these situations, when researchers have access to distancing data, COVID-19 cases do not appear to be statistically more common in schools with 3-foot distancing requirements than in those with 6-foot rules. But these studies did not or could not always look closely at specific distances in classrooms and their relationship to COVID-19 incidence, or how well students wore their masks. In addition, they were conducted before the uber-contagious Delta variant swept across the country, as well as during a time when schools were quite vigilant about their COVID-19 mitigation strategies, which may no longer be the case. The CDC states, “Many state, tribal, local, and territorial agencies are planning to or already have reduced prevention strategies, such as physical distancing and masking, for community settings including schools. Therefore, the 2021–2022 school year will not be directly comparable to the 2020–2021 school year.” 

The CDC, American Academy of Pediatrics, and other organizations have urged that all children return to school full-time. “There’s really this focus on making sure all students have the opportunity to get back to full in-person learning, because we know how critical that is,” says pediatrician Sara Bode at Nationwide Children’s Hospital in Columbus, Ohio, and chair-elect for the American Academy of Pediatrics Council on School Health. “I do think that sometimes there’s balance between all the layered mitigation measures and getting students back to class. If maintaining three feet of distancing means you can’t open your schools or get all the students back to class, then it’s likely worth going forward and getting all students back to class, with layering [other mitigation strategies] so it’s safe.” 

Challenging that goal is Delta, which has triggered a surge in the US that is far from over. The country posted 148,222 new cases of COVID-19 in kids for the week ending October 7, on par with the case numbers in kids during the surge last winter. “The risk [from COVID-19] is lower in children, but far from zero,” says Schaffner. “There is long COVID,”—which affects up to one in seven children who get COVID, according to one new study—“there is the multi-system inflammatory syndrome, there are children who have died of COVID.”   

See “SARS-CoV-2 Antigens Leaking from Gut to Blood Might Trigger MIS-C

Masks make all the difference

Requiring quarantine of people who have been within 6 feet of an infected person comes from research on other respiratory illnesses, based on the distance larger droplets carrying viruses can travel through the air. “When you’re talking to somebody, if you’re not wearing masks and are in the same room, the farthest droplets go [is] about five feet, which is why the distance rule is six feet,” says biologist Alicia Zhou, chief science officer at Color Health, Inc, in Burlingame, California, where she develops models of COVID-19 transmission in schools. “That’s not something unique to COVID, that’s just well-known about infectious diseases and droplet transmission.” 

See “Ferret Study Reinforces Role of Aerosols in SARS-CoV-2 Spread

Early in the pandemic, however, it became clear that SARS-CoV-2 wasn’t just transmitted by large droplets that quickly fall out of the air; it also travels on smaller droplets and as aerosol particles that remain suspended in the air for minutes to hours after being exhaled by an infected person, allowing it to spread and even travel on air currents around the room. The viral concentration tends to decrease the farther you get from the infected person, though coughing and sneezing can catapult the virus much farther than six feet. A recent paper on the evidence behind COVID-19 public health guidance reports, “the evidence base for recommending 6 feet of interpersonal spacing in the current pandemic is poor. Although it may be argued that 6 feet is practicable, there is no reassurance that infectious risk is negligible at that distance.” But, when all the people in a room wear a mask, “that changes drastically,” says Zhou. “The mask is keeping the person who’s infected from emitting droplets in the first place, but it’s also protecting the person who’s not infected. If any droplets do come through, they land on the mask, not on their nose or mouth. Distancing is less important if mask-wearing is very enforced.” 

The CDC quarantine distance guidance necessarily takes masks into consideration. Masks cut down transmission, even when students are physically close, according to the studies on which the CDC based its updated school distancing guidance last March that children can safely sit as close as 3 feet apart in schools as long as mask use is universal. (The definition of a close contact—someone who has to quarantine after exposure to an infected person—remained at six feet until August.) 

The mitigation layer of universal masking can be really effective even when you can’t completely distance to be safe and reduce transmission.

—Sara Bode, Nationwide Children’s Hospital

In a North Carolina study, for example, 4,969 students and staff (out of more than 100,000) in 13 school districts came into school with COVID-19 between October 2020 and February 2021. Of their close contacts in the schools, 209 subsequently tested positive. Mask use was universal, and in the elementary schools there was minimal physical distancing. The researchers reported that they “did not observe substantial differences in school-acquired cases between elementary, middle, and high school,” despite greater distancing in the upper grades. The CDC writes in its Science Brief that this study demonstrates low in-school transmission, even when schools weren’t extraordinarily vigilant about physical distancing. 

Similarly, in elementary schools in Salt Lake County, Utah, from December 2020–January 2021, there were 51 community-acquired cases of COVID-19 in 10,171 students and 1,214 staff, with five secondary cases likely resulting from in-school exposures, the CDC reported in its weekly digest. Distancing was 3 feet, and mask use was high. Other CDC reports and studies in districts with various distancing requirements plus universal mask use also found limited transmission within schools. In most of these studies, physical distance between students wasn’t precisely quantified, which made it difficult to know how close students who developed COVID-19 were to each other. 

“What that tells us is that the mitigation layer of universal masking can be really effective even when you can’t completely distance to be safe and reduce transmission,” says Bode.

The CDC writes in its Science Brief, “the preponderance of the available evidence from United States schools indicates that even when students were placed less than 6 feet apart in classrooms, there was limited SARS-CoV-2 transmission when other layered prevention strategies were consistently maintained.” The CDC declined to make a representative available for an interview with The Scientist, so it is not clear whether it used the same reports in changing the quarantine distance from 6 to 3 feet as it had in making the earlier change to the physical distancing requirement. 

Does Delta change everything?

The big concern is that these data are from last school year, before the Delta deluge, and during a time when families and schools were more vigilant about keeping kids distanced from each other and masked, says Brandon Guthrie, an epidemiologist at the University of Washington—a caveat also mentioned by the CDC in its Science Brief. Will the quarantine rule change affect spread in schools exposed to Delta? 

Anytime you make a mistake with masking with Delta, you’re going to pay a higher price.

—Daniel Benjamin, Duke University

The data are starting to come in. In a classroom of elementary students in Marin County, California, an outbreak of COVID-19 illustrates the importance of distance in the virus’ spread, but showed that without masks, even 6 feet wasn’t enough to keep students safe. In May and June of this year the teacher, who was unvaccinated, took off her mask a few times to read the children a story (all children were masked). After she tested positive for COVID-19, 12 of her students did as well, including 80 percent of the students in closest two rows (who were 6 to 12 feet from the front of the room and the teacher’s desk) and 28 percent of the students in the back three rows, at least 12, 18, and 24 feet away, respectively. The virus that caused the outbreak was genotyped, and it turned out to be the Delta variant; most sequences were indistinguishable from one another. 

“Anytime you make a mistake with masking with Delta, you’re going to pay a higher price,” says pediatrician Daniel Benjamin of Duke University, leader of the North Carolina studies on COVID-19 in schools. 

Over the summer, as the Delta variant became dominant, Benjamin’s research team in North Carolina looked at within-school transmission in 20 districts with universal masking but no set distance requirement (3 feet was recommended). The researchers found that Delta increased the proportion of secondary infections relative to primary infections. Before Delta, it was about 1:20; with the more infectious Delta variant, it’s about 1:13. There were 808 infections brought into the school from the community, which likely caused 64 school-acquired COVID-19 cases. 

In classrooms, kids sometimes pull down uncomfortable masks to sneeze, or hang their masks off their chin when the teacher isn’t looking. And since kids obviously can’t eat with masks, “Lunch is a high-transmission time,” says Benjamin. “The school districts that are masking, and doing a pretty reasonable job, a lot of them are getting considerable transmission at lunch.” In all these cases, “spacing probably is relevant. The more space there is when you have imperfect mask usage, the lower the potential risk would be,” says Guthrie. But when it comes to the CDC’s recent guideline change, “there’s nothing particularly magic about 3 feet versus 6 feet,” says Benjamin. “I think if both entities are masked, transmission is going to be low. If they’re unmasked, they’re probably at risk if they’re in the same classroom, as evidenced by the Marin County data.” 

Test and stay

In addition to masking and distancing, another layer of protection is in-school rapid testing. Massachusetts, for example, doesn’t send kids home if they were within 3 feet of an infected person in a classroom, as long as they opt into a Test and Stay program that requires them to get tested every morning with a rapid antigen test; there is no similar testing program for students in quarantine. The students stay in school unless they develop symptoms or test positive. (Masking is mandatory in Massachusetts.) Data from the most recent available week, October 7 to October 13, show 1,901 student COVID-19 cases; 106 of these were close contacts of infected students, and they tested positive through the Test and Stay program (there are about 920,000 students in Massachusetts schools). A representative from the state tells The Scientist that these 106 students may have caught the disease from the primary cases in school, or from other contacts outside school. The state reports that the program saved 10,898 in-person days of school that week. “A positive antigen test is a very good marker of whether or not somebody is infectious,” says Westyn Branch-Elliman, an infectious diseases specialist and clinical researcher at the VA Boston Center for Healthcare Organization and Implementation Research. Students are unlikely to be infectious when they test negative, even if they go on to test positive later, she says. “Test to Stay is very powerful because it keeps symptomatic people from spreading to their peers.”

Most experts agree that, especially with Delta, vaccinating adults should be the first priority, especially since the youngest children cannot yet receive vaccinations. “If we are that concerned about the kids, how is it that all these concerned adults are not vaccinated? Because that’s the single most important thing in any community,” says Schaffner. “If a community had one hundred percent of its adults vaccinated, believe me, the risk in schools would be tiny.”