MONTREAL, October 18, 2021 – During COVID-19, in-class vs. remote teaching is a central concern of families, educators and decision makers. Dr. Caroline Quach-Thanh and her research team at CHU Sainte-Justine considered a related issue: the optimal use of rapid testing to contain outbreaks in schools, in addition to evaluating the efficacy of rapid antigen tests for COVID-19. The results indicate that the use of polymerase chain reaction tests, known as PCR tests, in symptomatic individuals remains the most sensitive technique, yet rapid tests have a role to play when children present with symptoms.

Rapid tests have been used for years to diagnose respiratory pathogens such as influenza. They are relatively inexpensive and can be used directly at the point of care. Their performance characteristics vary, but they generally have high specificity and moderate clinical sensitivity compared to PCR tests.

"Clinical sensitivity refers to the probability that the test will detect an infection in people who are sick. Specificity refers to the probability that the test will confirm that people who are not sick do not have an infection. For SARS-CoV-2, the specificity of the rapid tests was quite high, but the sensitivity was quite low, at 28.6% in asymptomatic individuals and 83.3% in symptomatic individuals," explains Dr. Quach-Thanh, clinician-researcher at CHU Sainte-Justine and Professor in the Departments of Microbiology, Infectiology and Immunology and of Pediatrics at Université de Montréal.

In other words, compared to PCR tests, antigenic rapid tests risk missing some cases of COVID-19. However, this risk is lower in symptomatic individuals.  Furthermore, rapid antigen tests work best when the viral load is high, which is also when the contagion is most likely. So, there is a role for these tests, "especially when children have symptoms from a host of other viruses," indicates Dr. Quach-Thanh.

The study involved the participation of over 2000 high school students and nearly 300 staff members recruited from two Montreal high schools who were followed between January and June 2021.

Twenty-five percent of asymptomatic participants were tested weekly with rapid and PCR tests, both of which were self-administered. All symptomatic participants were also tested using the two methods studied.

Although the number of outbreaks in the two participating schools was not lower than in other high schools in the Montreal area, we found a higher proportion of asymptomatic cases in our participating schools. This indicates that cases were likely missed in other schools in the area, serving as a source for additional potential outbreaks. Moreover, although there had always been a concern about transmission of COVID-19 in schools, the study allowed us to see that, of the cases with a known source, 72.5% were a consequence of household transmission, and 25% were due to transmission within the school.

According to Dr. Quach-Thanh, "rapid tests are of little use as a screening tool for asymptomatic individuals, given the resources required and their low sensitivity in this setting. They should be reserved primarily for symptomatic individuals." 

"It therefore seems preferable to strengthen triage policies (symptoms and contact history) and to use rapid antigenic tests for symptomatic individuals in schools in order to prevent the spread of the disease," she concludes.

This study was funded by the ministère de la Santé et des Services sociaux du Québec.

– 30 –