Schools in Sweden emphasized hand cleaning, but neither students nor teachers wore masks.
Martin von Krogh / Getty Images
By Gretchen Vogel
ScienceThe COVID-19 reporting is supported by the Heising-Simons Foundation.
A careful analysis of health data from Sweden indicates that schools with only minimal precautions are kept open in the spring, and that the risk of teachers being diagnosed with the pandemic coronavirus has approximately doubled. Their partners had a 29% higher risk of becoming infected than partners of teachers who moved to teaching. Parents of children at school were 17% more likely to be diagnosed with COVID-19 than those whose children learned at a distance.
The question of whether the damage of school closure outweighs the risks of virus transmission in classrooms and corridors has been the subject of intense debate worldwide. Outbreaks have shown that the virus can spread at least occasionally via schools to the wider community, and some data suggest that teachers have a higher-than-average risk of infection. However, it was difficult to separate the transfer of the school from other confusing factors, especially as schools tended to open or close in conjunction with other restrictions being lifted or tightened.
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In the same week as new guidelines for opening schools of the U.S. Centers for Disease Control and Prevention, the new study will help policymakers better understand and weigh the risks and benefits. “It’s great to see such a carefully conducted study,” said Anita Cicero, an expert on pandemic response policy at Johns Hopkins University’s Bloomberg School of Public Health. ‘We are hungry for studies’ that quantifies the impact of open or closed schools on wider community distribution.
In March 2020, schools around the world will close when governments try to keep SARS-CoV-2 in check. But children in Sweden up to the ninth grade continued to attend the class, while the pupils in the 10th to the 12th grade switched to distance education. Economists Jonas Vlachos, Helena Svaleryd and Edvin Hertegård at Uppsala University used this natural experiment and Sweden’s detailed healthcare data.
They compared infection rates of parents whose youngest child was in the ninth grade with those whose youngest was in the 10th grade. They also compare infection rates among teachers who continued to teach in private at lower secondary schools (grades seven to nine) with those of teachers at high schools (grades 10 to 12) who taught at a distance. Finally, they compare infection rates among the spouses of teachers in the two types of schools. They describe their results in a paper presented on 12 February 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 health workers from the study because they had more exposure to the virus and were tested more frequently. Sweden’s coronavirus testing was very limited in the spring and only drove people with moderate to severe symptoms. Although it missed many cases, Vlachos says, it was actually a benefit to their analysis. As the tests increased in the summer and fall, the test figures began to correlate more with the income, which would have skewed the findings. (So few children and teens were tested that the researchers could not draw conclusions about their infection rates.)
The authors say that the impact on teachers was significant, and the results highlight the need to prioritize educators in COVID-19 vaccination schedules. While teachers at high school schools had an average risk of infection among 124 occupations in Sweden, the researchers found that teachers ranked lower in lower secondary rank. (Primary school teachers have a somewhat lower but still above-average risk.)
Among the country’s 39,000 teachers in primary schools, 79 were admitted to hospital with COVID-19 between March and June, and one died. If the schools were to move to online learning, the authors could avoid perhaps 33 of the serious cases.
The addition of masks would likely have reduced the risks for both teachers and families, says Danny Benjamin, a pediatrician at Duke University who studied the spread of the pandemic coronavirus in schools in North Carolina. But the Swedish study shows that ‘even if schools do not need masking, the risk for families of personal schooling is low’, he says.
Vlachos agrees that more interventions will further reduce the risk. “Our estimates are probably an upper limit,” he says.
The authors calculated that the retention of primary schools probably led to 500 additional cases in the spring among the 450,000 parents with children in primary school and 38 additional cases among teachers’ partners. (Because the test was so limited, the actual number of additional infections was probably much higher, the authors say.)
“The results for parents provide perhaps the best evidence of how school closures affect the transmission of viruses into society,” said Douglas Almond, an economist at Columbia University. By comparing families with ninth and tenth pupils, the team was able to compare families with teens whose social behavior and virus risk were similar, he says. “This is where their natural experiment really shines.” Jonas Björk, an epidemiologist at Lund University, also has a very elegant ability to connect teachers to their spouses through the health registries.
“It is to be expected that the opening of schools could increase COVID-19 infections, but knowledge is not the policy,” Almond said. ‘One must know how much infections are increasing as a result of the reopening of the school. This is the best paper I know that quantifies this effect. ‘
More comparisons between schools with different policies regarding masks, distance and quarantines will be helpful, Cicero says. Using the Swedish health register, the researchers were able to take the analysis even a step further and look at the risk to parents of teachers, says Björk, who will help 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 the transfer of schools are even more important, says Benjamin. Cicero agrees. “This is the next step” for research, she says: Funding studies on the impact of the variants and which interventions can keep the risks at schools as low as possible.
Correction, 16 February 2021, 02:35: This story has been corrected to say that the study can be extended to parents of teachers, not grandparents of students.