In DepthCOVID-19

Data in paper about Swedish schoolchildren come under fire

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Science  05 Mar 2021:
Vol. 371, Issue 6533, pp. 973-974
DOI: 10.1126/science.371.6533.973

Pediatrician and epidemiologist Jonas Ludvigsson of Sweden's Karolinska Institute has been a staunch defender of his country's unorthodox coronavirus policies. Among them was the decision in the spring of 2020 to keep preschools and schools open for children through grade nine, despite limited understanding of the virus and with few precautions to prevent school outbreaks. But Ludvigsson's research, which suggested that policy was relatively safe—and has been widely cited in arguments against school closures—has repeatedly come under fire from critics of Sweden's approach.

The latest example is a research letter, published online by The New England Journal of Medicine (NEJM) on 6 January, that looked at severe disease and deaths among children and teachers in Sweden between March and June 2020. Critics—including the authors of two letters NEJM published on 1 March—have said the study was beside the point and a distraction. It's well known that children are less likely to be hospitalized or die from COVID-19; instead schools worldwide have shut down to slow the spread of the virus in the wider community.


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A barrage of criticism led Jonas Ludvigsson of the Karolinska Institute to quit COVID-19 research.

PHOTO: ALEXANDER DONKA

But Science has learned that another complaint sent to NEJM makes a more serious allegation: that the authors deliberately left out key data that contradicted their conclusion.

The complaint comes from Bodil Malmberg, a private citizen in Vårgårda, Sweden. She used the country's open records law to obtain email correspondence between Ludvigsson and Swedish chief epidemiologist Anders Tegnell, the architect of the country's pandemic policies, that shed light on how the paper came about. Malmberg says she requested the emails because the data in the NEJM paper “did not add up.” Ludvigsson does not dispute the content of the emails, but stands by the study's conclusions. However, he says the barrage of criticism and personal attacks has made him decide to quit COVID-19 research.

Ludvigsson, whose prepandemic research focused on gastroenterology, was one of the 47 original signers of the Great Barrington Declaration, a controversial document published in October 2020 that argued that pandemic policies should focus on protecting the vulnerable while the rest of the population builds up immunity through natural infection.

Ludvigsson's research seemed to support those ideas. In a review about children's role in the pandemic, published in Acta Paediatrica in May 2020, he reported there had been “no major school outbreaks in Sweden,” which he attributed to “personal communication” from Tegnell. But as critics noted, Swedish media had reported several school outbreaks by then, including one in which at least 18 of 76 staff were infected and one teacher died. (Children were not tested.)

His NEJM letter sounded another reassuring note. It reported that in all of Sweden, only 15 children, 10 preschool teachers, and 20 school teachers were admitted to intensive care units for COVID-19 complications between March and June 2020. The authors noted that 69 children ages 1 to 16 died of any cause in Sweden during that same period, compared with 65 between November 2019 and February 2020, suggesting the pandemic had not led to an increase in child deaths.

But the emails obtained by Malmberg show that in July 2020, Ludvigsson wrote to Tegnell that “unfortunately we see a clear indication of excess mortality among children ages 7-16 old, the ages where ‘kids went to school.’” For the years 2015 through 2019, an average of 30.4 children in that age group died in the four spring months; in 2020, 51 children in that age group died, “= excess mortality +68%,” Ludvigsson wrote. The increase could be a fluke, he wrote, especially because the numbers are small. Deaths in 1- to 6-year-olds were below average during the same period, so combining the age groups helped even out the increase, he noted.

The emails “cast a serious shadow” on the research letter, Malmberg wrote in an email to NEJM. (The journal declined to comment on her complaint.) Epidemiologist Jonas Björk of Lund University agrees that the time comparison used in the paper was unusual. “I can see no good reason to compare with previous months,” he says. “It is standard to compare with the same period in previous years” to account for seasonality and to decrease statistical uncertainty.

Ludvigsson told Science a peer reviewer for NEJM suggested the comparison to deaths in November through February, and that he combined the numbers for preschool and school children because of NEJM's length requirements. As part of his 1 March response to the published critiques, he updated the paper's supplementary data with the monthly deaths from 2015 through 2020, but did not flag the 68% increase in school-aged children.

The rise in mortality is unlikely to be due solely to COVID-19, Björk notes. Although Sweden tested very few children in the early months of the pandemic, fatal cases of COVID-19 would have likely shown up somewhere in the health care system. “This of course must be ruled out by looking at causes of death and medical records in more detail,” he says. And Björk agrees that the increase could be due to chance.

The mortality questions aside, critics say the NEJM letter just wasn't very helpful in the heated debate over school closures. The main concern is not that children may end up in intensive care, but that schools may accelerate community spread, says Antoine Flahault, a global health expert at the University of Geneva and a co-author on one of the critiques in NEJM. Nisha Thampi, a pediatric infectious disease physician at the CHEO pediatric health and research center in Ottawa, Canada, agrees. The paper “doesn't really tell me much about what's happening to kids in schools,” she says.

Arguing about whether schools are “safe” has slowed down efforts to find ways to reduce the risk of viral spread in classrooms and hallways, Flahault says—which is key to keeping schools open and the virus under control.

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