The United Nations estimates that more than 168 million children missed out on in-classroom learning as schools remained largely shut for almost an entire year due to coronavirus lockdowns. And in the United States, all 50 states recommended or mandated public school closures, affecting at least 124,000 schools and 55.1 million students.

As of April 2021, about 40% of U.S. public school students were still unable to attend schools in person full time — leading public health officials, parents and educators to worry that school children would fall behind both academically and developmentally.

In an effort to inform public policy on the school re-opening debate, a team of researchers at Stanford University, Harvard University, Massachusetts General Hospital and University of Maryland School of Medicine developed a model to simulate transmission in elementary and high school communities, as well as household interactions.

In a study published June 8 in the Annals of Internal Medicine, the researchers found that with controlled transmission of COVID-19 in communities and adherence to school-based prevention measures, elementary schools can open with few in-school transmissions. For high schools, and for elementary schools in communities with high COVID-19 rates, regular screening of students and staff without symptoms can reduce risk.

“Debates around school reopening have been incredibly contentious, because schooling has such central importance to students, families and society, but also because evidence on safety in schools during the pandemic has been incomplete and sometimes contradictory,” said Stanford Health Policy’s Joshua Salomon, a co-author of the study. For example, the researchers noted that many studies have found little secondary transmission in schools — yet coronavirus clusters in schools have also been documented in other well-regarded studies.

Despite the contradictions, there is little debate over the benefits of in-person education. There have been reports of high levels of absenteeism in the last year, increased depression, anxiety and thoughts of suicide. “Beyond educational and mental health outcomes, opening schools also improves access to social services for children and labor market outcomes for working parents, especially women,” the co-authors write.

“Returning to safe, full-time in-person schooling for every student in the fall – and keeping schools open and safe – should be a national priority. Our main motivation in undertaking this study was to define the conditions and actions that are needed to make that possible,” said Salomon, PhD, a professor of medicine and senior fellow at the Freeman Spogli Institute for International Studies.

The researchers simulated settings based on average U.S. classrooms, with elementary schools of 638 students from 500 households, and high schools of 1,451 students from 1,225 households. They allowed for daily local incidence to vary between 1 and 100 cases per 100,000 students, faculty, staff and adult household members. They then focused on the risk that transmission would occur on a school campus and spread to household members — a less explored area of research.

They found that community transmission rates are major drivers of in-school transmission. Whereas elementary schools are likely ‘mirrors’ of these community rates, high schools may be ‘amplifiers’ of community transmission. However, preventive measures in schools, such as masking, distancing, and ventilation, can strongly counter the impact of high community transmission. If schools have the resources to implement mitigation measures well, these measures not only reduce the number of infections transmitted in schools on average, but also substantially reduce the chance of worst-case outcomes, including large outbreaks.

“We predict that most in-school transmission will occur in the classroom during sustained contact, and interventions that reduce classroom transmission can be highly effective, including distancing, masking, or reducing class sizes,” the researchers concluded. The paper emphasizes that, even in the event of a fall COVID-19 resurgence, staff vaccination markedly decreases risk of transmission and illness.

The study also reports the potential benefits of weekly screening for students and staff without symptoms. Screening has two major benefits — both reducing the risk that people who are infected will come to school and infect others, as well as providing better real-time information on how much transmission is occurring in schools. As the authors note, data from screening programs can provide a more direct and current guide to local decisions than broader measures of cases rates in the community.

Salomon and his colleagues look ahead to the continuing rollout of COVID-19 vaccination for adults and teenagers to provide a major boost to prospects for safe reopening in the fall, at the same time that they acknowledge concerns over new, more virulent variants of COVID-19 that have been spreading widely in Britain, India and other areas.

“This shift highlights the urgent need for data and underscores the added value of routine asymptomatic screening,” the authors write. “Tradeoffs are inevitable between school disruption, risk for in-school transmission, and resources required to implement reopening strategies.”