While the rapid antigen tests remain a useful tool for the detection of COVID-19 infections, some of these diagnostic kits may be less sensitive to the variants of concern, according to a US study.

Scientists at the Harvard T.H. Chan School of Public Health and Beth Israel Deaconess Medical Center (BIDMC) in the US noted that the rapid tests were developed for use with the original SARS-CoV-2 viral strain that emerged late in 2019.

Since then, the virus has evolved countless times, and several viral variants of concern have emerged, including the highly infectious Delta and Omicron variants that swept across the world last summer and winter respectively. In the study published in the Journal of Clinical Microbiology, the researchers used live virus culture to assess how well four rapid antigen tests are able to detect these COVID-19 variants of concern.

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

Using three strains of cultured live virus, the team assessed differences in the limits of detection (LoD) -- the smallest amount of viral antigen detectable at 95 per cent certainty -- of four commercially available rapid antigen tests; the Binax, CareStart, GenBody and LumiraDx tests. The researchers found that all four tests were as sensitive to the Omicron variant, if not more, as they were to the original SARS-CoV-2 viral strain, known as WA1.

However, three tests showed less sensitivity to the Delta strain, with only the CareStart demonstrating equal detection of all three strains, they said. "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," said Kirby, also a professor of pathology at Harvard Medical School.

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