Rapid COVID test not keeping pace with new variants, scientists say

A new study has found that rapid antigen tests, which can be self-administered at home to monitor for COVID-19 infection, may not be keeping pace with emerging variants of concern that differ from the original strain of SARS-CoV-2.

The study, conducted by scientists out of the Harvard T.H. Chan School of Public Health and Beth Israel Deaconess Medical Centre (BIDMC), found that some rapid antigen tests may not be as effective at detecting different COVID-19 variants.

While rapid antigen tests remain a useful tool in detecting COVID-19 infections, the study's findings suggest these tests will likely need "continuous assessment" and updating when taking into account virus variants.

The findings were published Thursday, in the Journal of Clinical Microbiology.

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The main issue with rapid tests, scientists say, is they were developed with the original SARS-CoV-2 viral strain that emerged late in 2019. Since then, the virus has evolved numerous times and several viral variants of concern have emerged.

"Unlike the sensitive molecular tests that detect multiple SARS-CoV-2 genes, rapid antigen tests target a single viral protein," said co-corresponding study author and BIDMC clinical director James Kirby in a press release. "As the pandemic continues, however, some hypothesize that the performance of available antigen test may vary among the COVID variants of concern."

Using strains of live virus culture, the study's authors assessed how well four commercially availably rapid antigen tests were able to detect two COVID variants of concern -- the highly infectious Delta and Omicron variants.

According to the study, the team looked at the differences in the limits of detection, which is the smallest amount of viral antigen detectable at 95 per cent certainty, of the the Binax, CareStart, GenBody and LumiraDx rapid tests.

Researchers found that all four tests were sensitive to the Omicron variant, "if not more" so than to the original SARS-CoV-2 strain. However, three of the four tests showed less sensitivity to the Delta variant, with only the CareStart test showing "equal detection" of Omicron, Delta and the original virus.

Given the differences in the tests' limits of detection, Kirby, who is also a professor at Harvard Medical School, says this could have resulted in cases being missed amid the last two waves of the pandemic.

"We expect that the observed loss in Delta sensitivity could have resulted in a 20 per cent or more loss of detection in potentially infectious individuals – nevertheless, the most infectious individuals still should have been detected," Kirby said in the release.

"However, our findings suggest that antigen test performance needs to be re-evaluated for emerging variants to ensure they still meet the intended public health testing goals of the pandemic," he added.

Given the unknown sensitivity of rapid tests in relation to COVID-19 variants, experts have previously advised Canadians take two rapid tests to confirm whether or not they are infected.