Montreal study shows rapid COVID-19 tests have limited effectiveness in schools

Montreal researchers looking at the effectiveness of rapid COVID-19 tests in two schools have found the tests aren’t effective when used randomly as a preventive tool, but can be useful in situations where outbreaks are already suspected.

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Dr. Caroline Quach, a pediatric infectious diseases specialist and medical microbiologist at Ste-Justine children’s hospital, led a study which tested students at Calixa-Lavallée public high school in Montreal North and the Pensionnat du Saint-Nom-de-Marie private high school in Outremont.

The goal was to see how effective the tests would be in a high school setting.

Opposition politicians at the National Assembly, teachers’ unions and others have accused the province of letting the tests sit on a shelf rather than deploying them widely.

“Our conclusion is that just testing randomly children as they come into school is probably not the best use of your time and energy,” Quach said in an interview Tuesday.

Rapid tests miss cases in asymptomatic people

Quach said out of 4,500 children tested, only five positive cases were detected, a much smaller number than would be expected.

She said rapid tests are less sensitive than the diagnostic tests currently used at testing centres, also known as PCR (polymerase chain reaction) tests.

Quach said that means the rapid tests are more likely to miss COVID cases in people with no symptoms.

“When you’re asymptomatic your viral load is usually lower, and these tests are less sensitive than PCR,” Quach said.

“So if you were to cast a very wide net among people who maybe would have a low viral load, the chances are that these tests would miss it,” Quach said.

Quach said the tests proved more effective when an outbreak was suspected.

“If you use them to test children and staff who have symptoms, then it’s actually pretty good at picking it up,” she said.

“We used them to test contacts within a classroom and so we were able to detect earlier those that would be COVID positive,” Quach said.

But Quach said using the tests randomly in a preventive way to find relatively few cases was inefficient.

“We did manage to pick some up cases, but the energy and time investment it took to detect some of those was immense,” Quach said.

Use tests in a targeted way

Quach said one way the tests could be useful is in elementary schools next fall, where students won’t yet have been vaccinated.

She said the tests could be deployed when a student shows symptoms to quickly test other children and staff they’ve been in contact with to rule out COVID. But she said, even in that case, it would be best to follow up with more accurate PCR testing.

Quach also said the tests might be useful in controlled indoor settings such as concerts or restaurants.

“You could combine proof of vaccination with rapid tests at the door,” she said.

“That rapid test would at least allow you to remove those with a high viral load who could be asymptomatic,” she said.

According to Radio-Canada, as of last month, the province had a stockpile of more than four million rapid tests that weren’t being used.

CBC