Large-scale randomized testing and contact tracing over time, which would give a more complete picture of who transmits the virus and how, hasn’t been done yet in schools. In July, in the journal Emerging Infectious Diseases, researchers from the Korea Centers for Disease Control and Prevention published the results of tracing more than 59,000 contacts of 5,706 coronavirus patients. Children younger than 10 were found to have transmitted the virus much less than did those between 10 and 19, whose transmission rate was equivalent to that of adults. But only 3 percent of patients in that initial cohort were 19 and younger, and their having been tested probably means they presented symptoms. It’s still unclear how asymptomatic children, who are hard to identify, might spread the virus; it’s also unclear if there are differences in transmission between the ages of 10 and 19.
“A lot of the data we’re getting from different sources is messy and not necessarily pointing in the same direction,” says Nicholas Davies, an epidemiologist at the London School of Hygiene & Tropical Medicine. He and colleagues used a statistical method called Bayesian inference to test several hypotheses about children and Covid. These included the premises that kids are being infected but not showing symptoms and that kids are less susceptible to infection. Based on epidemiological data from China, Italy, Japan, Singapore, Canada and South Korea, the researchers concluded that both premises were probably true, to an extent. Their findings, published in Nature Medicine in June, estimate that people under 20 are about half as likely as older age groups to become infected, and that among 10- to 19-year-olds who do get the virus, only 21 percent will have clinical symptoms. They couldn’t make finer age distinctions, nor say how likely any children are to infect others.
None of these studies directly addresses the impact of reopening schools on the spread of Covid. In fact, when researchers from the University of Washington departments of global health and epidemiology began compiling a summary of models from 15 other countries where students have returned, they found “very few” scientific publications on the topic and relied primarily on news reports. In nearly all countries, they observed, schools adopted safety measures, including face masks and social distancing. None of the countries (except Sweden, which kept many schools open) resumed classes before national rates of infection had significantly declined; there is no evidence to say what the outcome of opening schools would be in areas of the U.S. where the virus is surging. In Germany, where infection rates were higher than in other European countries, the return of older students accompanied an increase in infections among one another but not staff, according to a preprint led by researchers at the University of Manchester and Public Health England.
In Israel, students and staff wore masks after schools reopened in early May. But several weeks later, those rules were relaxed. According to Haaretz, outbreaks began soon after, exposing thousands at schools to infection, causing many of them to close down again. There’s “not clear cause and effect” between the removal of masks and the outbreaks, says the summary’s lead author, Brandon Guthrie, but it’s “circumstantial evidence” that they offer some protection in classrooms. It also reveals how unenforceable the health guidance schools are receiving can be.