Rapid COVID tests used twice weekly could detect most contagious kids

April 12, 2021

A new USC study found that rapid COVID-19 tests show promise for schools, especially if they use serial testing, such as twice a week, according to a news release from the university.

“Serial testing is critical because one-time antigen tests might not identify asymptomatic children at or shortly after the onset of infection,” said Neeraj Sood, PhD, Professor of Health Policy, Preventive Medicine and Business at the USC Schaeffer Center for Health Policy and Economics. “But serial testing will likely identify these children as they subsequently develop high viral loads and become infectious a few days later.”

The study was published in PLOS ONE.

To see how well the inexpensive, easy-to-use tests might work as a screening tool for schools, Sood and his colleagues launched their study at a walk-up testing site in Los Angeles County during the winter surge of COVID cases.

Children arriving at the testing site with their parents were given the opportunity to be tested for COVID twice – once with a rapid antigen test and once with the “gold standard” PCR test – to confirm the results of the rapid test. Overall, 774 children ages 5 to 17 years old took both tests.

The study found that the rapid antigen tests have very low false positive rates but a moderately high false negative rate. In other words, the tests rarely identified someone as positive who was not but did occasionally identify someone as negative who in fact had COVID.

Among asymptomatic children who had COVID, as measured by the PCR test, the rapid antigen test identified about 50% of COVID-19 cases. The reason the rapid antigen test failed to identify many COVID-19 cases in children was because the majority of children tested had low viral loads as measured by Ct values – the number of times one needs to amplify the test sample to detect the genetic material of the virus.

The test was over 90% accurate when focused on the sample of children with high viral loads who were very likely to be infectious. The test was about 99% accurate among asymptomatic children who did not have COVID-19. That is, the test gave a false positive result in 1 out 100 asymptomatic children.

Sood said the rapid tests, which require inserting a swab about a half-inch up the nose, could be administered by school staff or be self-administered with supervision. In addition, serial testing could allow schools to relax social distancing rules, he said.

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