Schools in Sweden emphasized keeping hands clean, but neither students nor teachers wore masks.
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By Gretchen Vogel
ScienceCOVID-19 reports are supported by the Heising-Simons Foundation.
A careful analysis of Sweden’s health data suggests that keeping schools open with just the minimum of precautions in the spring has almost doubled the risk of teachers being diagnosed with the pandemic coronavirus. Their partners faced a 29% higher risk of being infected than partners of teachers who started teaching online. Parents of children at school were 17% more likely to be diagnosed with COVID-19 than those whose children were in remote education.
Whether the damage from school closings outweighs the risks of transmitting the virus in classrooms and corridors has been the subject of intense debate worldwide. Outbreaks have shown that the virus can spread to schools in the wider community, at least occasionally, and some data suggest that teachers are at above-average risk of infection. However, it has been difficult to separate transmission at school from other confounding factors, especially as schools tend to open or close in conjunction with other restrictions lifted or tightened.
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Coming out in the same week as the new United States Centers for Disease Control and Prevention school opening guidelines, the new study will help lawmakers better understand and assess the risks and benefits. “It is great to see such a careful study,” says Anita Cicero, a specialist in pandemic response policy at the Johns Hopkins University Bloomberg School of Public Health. “We are hungry for studies” that quantify the impact of open or closed schools on the broader transmission of the community.
In March 2020, schools around the world closed as governments tried to control SARS-CoV-2. But children in Sweden up to the ninth grade continued to attend classes, while those in grades 10 through 12 switched to remote education. Economists Jonas Vlachos, Helena Svaleryd and Edvin Hertegård, from Uppsala University, took advantage of this natural experiment and Sweden’s detailed health data.
They compared the infection rates of parents whose youngest child was in the ninth grade with those whose youngest child was in the 10th grade. They also compared infection rates in teachers who continued to teach personally in high schools (7th to 9th grades) with those in secondary schools (10th to 12th grades), who taught remotely. Finally, they compared the infection rates in the spouses of teachers from both types of schools. They describe their results in an article published on February 12 in the Proceedings of the National Academy of Sciences.
The authors took steps to ensure that their groups were as comparable as possible. For example, they excluded families with healthcare professionals from the study because they had more exposure to the virus and were tested more frequently. The coronavirus test in Sweden was very limited in the spring, looking only at people with moderate to severe symptoms. Although this lost many cases, says Vlachos, it was actually an advantage for the analysis. As testing increased in summer and fall, test rates began to correlate more with income, which would have skewed the results. (So few children and adolescents were tested that researchers were unable to draw conclusions about their infection rates.)
The impact on teachers was significant, say the authors, and the results underscore the need to prioritize educators on the COVID-19 vaccination schedules. Although high school teachers have an average risk of infection among 124 occupations in Sweden, the researchers found, high school teachers came in seventh. (Primary school teachers had a slightly lower risk, but still above average.)
Among the country’s 39,000 high school teachers, 79 were hospitalized with COVID-19 between March and June, and one died. Moving these schools to online education would have prevented perhaps 33 of these serious cases, the authors estimate.
Adding masks would probably have reduced the risks for teachers and families, says Danny Benjamin, a pediatrician at Duke University who studied the spread of the coronavirus pandemic in schools in North Carolina. But the Swedish study shows that “even if schools do not require a mask, the risk for families to study in person is low,” he says.
Vlachos agrees that further interventions would further reduce the risk. “Our estimates are probably an upper limit,” he says.
The authors calculated that keeping high schools open probably led to an additional 500 cases detected in the spring among 450,000 parents with high school children and 38 additional cases among partner teachers. (Because testing was so limited, the actual number of additional infections was probably much higher, the authors note.)
“The results for parents provide perhaps the best evidence of how closing the school affects the transmission of the virus in society,” says Douglas Almond, an economist at Columbia University. When comparing families with students in the ninth and tenth grades, the team was able to compare families with teenagers whose social behavior and viral risk were similar, he says. “This is where their natural experiment really shines.” The ability to link teachers to their spouses through health records “is also quite elegant,” says Jonas Björk, an epidemiologist at Lund University.
“It is to be expected that the opening of schools can increase COVID-19 infections, but knowing that does not inform the policy,” says Almond. “You need to know how many infections increase due to the reopening of schools. This is the best article I know that quantifies this effect. ”
More comparisons of schools with different policies regarding masks, distance and quarantines would be helpful, says Cícero. Using the Swedish health record, researchers could even take the analysis a step further and look at the risk for teachers’ parents, says Björk, which would help to estimate the impact on a more vulnerable age group.
The emergence of more transmissible variants of SARS-CoV-2 means that masks and other interventions to prevent school transmission are even more important, says Benjamin. Cicero agrees. “This is the next step” for the research as well, she says: to fund studies on the impact of variants and what interventions can keep the risks in schools as low as possible.
Correction, February 16, 2021, 2h35: This story was corrected to say that the study could be extended to teachers ‘parents, not students’ grandparents.