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Covid-19: Daily testing of schoolchildren does not lead to substantial increase in cases, preliminary data show

BMJ 2021; 374 doi: https://doi.org/10.1136/bmj.n1878 (Published 26 July 2021) Cite this as: BMJ 2021;374:n1878
  1. Ingrid Torjesen
  1. London

Daily testing of secondary school pupils who have been exposed to a positive covid-19 case does not lead to a substantial rise in the number of covid-19 cases and could reduce covid related absences by as much as 39%, a pilot study shows.1

The Daily Contact Testing in Schools trial, funded by the Department of Health and Social Care and supported by the Department for Education and the Office for National Statistics, was set up to investigate whether daily testing was as safe as sending close contacts home to isolate and whether it would reduce absences. It is led by Tim Peto, professor of medicine at the University of Oxford.

The non-inferiority trial’s protocol specified that a relative increase in transmission of up to 50% was acceptable. The researchers concluded that they were 97.5% confident that, on the basis of participation rates in the trial, any increase in the rate of symptomatic infection in the intervention arm did not exceed 22% more than in the control arm, and if all people eligible to participate in daily testing did so it would not exceed 34%.

“In both analyses the point estimate favours a slight to modest reduction in incidence with the intervention,” they said. Their results are published in a preprint, and the study has not been peer reviewed.

Infection rates

The researchers randomly allocated 201 schools in England either to the control arm, where close contacts of positive cases isolated for 10 days, or to the intervention arm, where close contacts came to school and underwent daily lateral flow testing at a supervised testing site for seven days.

From 19 April to 27 June they found a rate of 59.1 symptomatic PCR confirmed infections in 100 000 a week among pupils and staff (657 during 7 782 537 days) at schools where contacts isolated, as compared with 61.8 in 100 000 a week (740 during 8 379 749 days) in schools where contacts continued to attend school with daily testing.

Rates of asymptomatic infection in contacts who provided a PCR sample were 1.5% (44/2981) in the intervention group and 1.6% (14/886) in the control group, and rates of symptomatic infection were 1.3% (79/5955) and 0.9% (44/4665), respectively.

The researchers suggested that, on the basis of recorded absences (1.8% in schools where contacts isolated; 1.5% in schools where contacts were tested), replacing isolation of contacts with daily testing could reduce pupil absences by as much as 39%.

Viral levels

Jon Deeks, professor of biostatistics at the University of Birmingham, noted that the study protocol gave no rationale for an increase of less than 50% in transmission being acceptable. “Many would consider much smaller increases that this trial cannot exclude to be unacceptable, particularly when event rates are high, as is currently the case,” he said.

Although absences were lower and covid infection rates higher in the schools with daily testing than with isolation, the differences were too small to give confidence that this had not been created by chance, said Deeks.

He explained, “Part of the reason for the difference being small was that contact testing ended up being possible in only around half the contacts allocated to use it—due to the children refusing testing, being absent, or the school being closed because of the size of the outbreak.

“Thus the study has failed to show that daily contact testing increases school attendance [or to] provide data to show that it is safe, but it has shown that lateral flow testing misses around half of the contacts who were infected by their friend at school.”

Previously there had been no data on the accuracy of the Innova/NHS lateral flow test in UK schoolchildren. This study used the Orient Gene test, which is thought to be more sensitive than Innova, and identified only 32 of the 60 cases in school contacts, so its sensitivity is only 53%, said Deeks.

“This illustrates the risk of using lateral flow tests, which can only detect high viral levels, to check whether contacts are infected,” he said. “As those that the test missed were close contacts, they are likely to have been in the early stages of infection when viral levels are rising fast and the students are becoming infectious.”

Paul Hunter, professor in medicine at Norwich School of Medicine, said that the researchers’ conclusion, that “there was no evidence that switching from isolation at home to daily contact testing increased rates of symptomatic covid in students and staff,” could not be used as proof that either intervention actually reduced the risk of transmission.

“The conclusions could equally be that both interventions are highly effective or that both interventions have no value whatsoever,” he said. “Nevertheless, I suspect that when schools return some form of continuing control will be required, and at least daily testing as in this study will have less harm on children’s education than the current exclusion policy, whether or not it has real benefit in controlling the epidemic.”

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