How easily can COVID-19 spread in a classroom?

School systems are struggling to open safely, but a risk assessment tool warns of high risk.

PHOTOGRAPH BY George Frey/Getty Images

On the first day of the second week of school in Cherokee County, a teacher looked out at her class and noticed that one boy wasn’t taking notes. As she walked over to see if he needed help, he clutched his head. He told her he had a throbbing headache.

Headache is a common symptom of Covid-19, so she quickly sent him to the school nurse. A few minutes later, he was back in the classroom. He didn’t have a fever, the nurse said, and the boy didn’t want to call his mother to take him home. (Just 43 percent of diagnosed COVID-19 patients report having a fever, according to a study by the Centers for Disease Control and Prevention.)

The boy stayed in the classroom. He didn’t wear a mask. Mask-wearing is encouraged but not required for students in Cherokee County, where several high schools have been temporarily shut down and more than 2,000 students have been quarantined. The teacher, who wore a mask, could only keep her distance and hope for the best.

School nurses follow their professional judgment, according to Barbara Jacoby, chief communications officer for the Cherokee County School District, who responded by email. “Our school nurses, who all are RNs or LPNs, are assessing student symptoms and making the determination on whether to send students home. They are medical professionals and very familiar with the symptoms of COVID-19,” she said.

But the incident reveals new tension in a usually festive rite of passage. This year, “back to school” brings more apprehension than excitement. “Every day, kids are either missing from class or they’re coming up to me and other teachers saying they don’t feel good,” said the Cherokee County teacher, who asked not to be identified.

Cherokee schools opened on August 3, putting them at the vanguard of in-person education in the era of COVID-19. (About a quarter of Cherokee students chose to enroll in virtual learning.) “We are continuously reviewing our protocols to determine additional improvements,” Jacoby said, noting that teachers have been provided face shields in addition to masks, if they want them. In an act of transparency, the district posts weekly COVID case counts per school, and it has trained teams of custodians in each school on how to properly disinfect surfaces.

As that county’s experience already demonstrates, schools will have to open “with a tremendous amount of precautions,” says Marybeth Sexton, a former high school biology teacher who is now an infectious disease physician and assistant professor at Emory University School of Medicine. “The school day won’t look like it normally does if it’s done safely.”

Administrators, teachers, parents, and students all agree there is no perfect solution, but how can they measure the risk of returning to school? The coronavirus spreads through droplets and possibly smaller aerosols when people talk, sing, cough, or sneeze, and one published report found it spread from recirculating air in a restaurant.  But how do those dangers apply in a classroom?

From a risk perspective, a school classroom is essentially a type of public gathering. Joshua Weitz, a professor of biological sciences at Georgia Tech, created an interactive online tool to help people gauge the risk of gatherings of different sizes, based on the most recent COVID stats by county. It takes into account an inherent undercount in the official state and county data. CDC conducted serology studies across the country—looking for antibodies that show whether someone has been infected with COVID—and estimated that the number of actual cases is about 10 times greater than the number reported. Weitz factors that into his risk assessment. (A user can lower the multiple to five if the county has a robust testing program. A high positivity rate may indicate that more testing needs to be done.)

In a group of 25—about the size of an average class—the model indicates there’s a 67 percent chance in Cherokee County that at least one person has COVID-19, according to the most recent data. (The odds drop to 42 percent in the more optimistic scenario of five actual cases per reported case.)

Without frequent, widespread, rapid testing, no one knows who that one person is. In schools, hundreds of different interactions take place—on buses, in hallways, in locker rooms, in cafeterias.

Refreshing the indoor air can dilute the virus. But even before the pandemic, studies found schools often have inadequate ventilation; a 2013 study of California elementary schools linked classrooms with poor ventilation to an increase in absence due to illness. In June 2020, the Collaborative for High Performance Schools, an organization that focuses on improving student health and learning through better school buildings, issued a report recommending improved ventilation rates and filtration as a way to improve indoor air quality and reduce the risk of spread of the virus. Some Georgia schools report that they are making changes to their HVAC. For example, Cherokee County is upgrading the air filters in its HVAC system to MERV-13, which are more efficient at filtering particles, says Jacoby, the school system spokesperson.

Opening a window—or, better yet, taking the class outside—makes the environment safer, but transmission from close contact can still occur, Sexton says. Weitz, who is founding director of Georgia Tech’s doctoral program in quantitative biosciences, says, “It’s unfathomable to me that they haven’t imposed mask mandates from the very beginning.”

Masks aren’t perfect, but by blocking the spread of droplets, they may reduce the viral load that individuals are exposed to, says Richard Rothenberg, an epidemiologist and Regent’s Professor of public health at Georgia State University. “Viral load is related to severity of illness,” he says.

As Cherokee County School Superintendent Brian Hightower stated when the system closed Etowah High School just eight days after opening: “As your Superintendent, I wear a mask whenever I cannot social distance. We know all parents do not believe the scientific research that indicates masks are beneficial, but I believe it and see masks as an important measure to help us keep schools open.”