Worcester, MA—The question that has come up often during the COVID-19 pandemic is whether the evolving SARS-CoV-2 variants affect the accuracy of rapid antigen tests (Ag-RDTs) in detecting infection.

A University of Massachusetts Chan Medical School–led study sought to compare the performance of Ag-RDTs in detecting the Delta (B.1.617.2) and Omicron (B.1.1.529) variants of SARS-CoV-2.

The investigation published in the Annals of Internal Medicine was a secondary analysis of a prospective cohort study that enrolled participants between October 18, 2021, and January 24, 2022. Participants took Ag-RDTs and collected samples for reverse transcriptase polymerase chain reaction (RT-PCR) testing every 48 hours for 15 days. While all Ag-RDTs were completed at home, nasal swabs for RT-PCR were shipped to a central laboratory for the parent study.

Of the 7,349 participants enrolled in the parent study, 5,779 asymptomatic persons who tested negative for SARS-CoV-2 on Day 1 of the study were eligible for this substudy. The researchers for the analysis measured the sensitivity of Ag-RDTs on the same day as the first positive (index) RT-PCR result and 48 hours after the first positive RT-PCR result.

The results indicated that 207 participants were positive on RT-PCR—58 Delta and 149 Omicron. The study team determined that differences in sensitivity between variants were not statistically significant (same day: Delta, 15.5% [95% CI, 6.2%-24.8%] vs. Omicron, 22.1% [CI, 15.5%-28.8%]; at 48 hours: Delta, 44.8% [CI, 32.0%-57.6%] vs. Omicron, 49.7% [CI, 41.6%-57.6%]).

In addition, among 109 participants who had RT-PCR-positive results for 48 hours, rapid antigen sensitivity did not differ significantly between Delta- and Omicron-infected participants (48-hour sensitivity: Delta, 81.5% [CI, 66.8%-96.1%] vs. Omicron, 78.0% [CI, 69.1%-87.0%]).

In fact, the research pointed out that only 7.2% of the 69 participants with RT-PCR-positive results for shorter than 48 hours tested positive by Ag-RDT within a week, while those with Delta infections remained consistently negative on Ag-RDTs.

“The performance of Ag-RDTs in persons infected with the SARS-CoV-2 Omicron variant is not inferior to that in persons with Delta infections,” the authors concluded. “Serial testing improved the sensitivity of Ag-RDTs for both variants. The performance of rapid antigen testing varies on the basis of duration of RT-PCR positivity.”

Background information in the study noted that rapid antigen tests have lower sensitivity than RT-PCR tests for detecting SARS-CoV-2, although sensitivity can be improved through serial testing. Previous studies tend to predate the emergence of the Omicron (B.1.1.529) variant.

Complicating the issue even more is that certain mutations might cause protein conformational changes that affect the target binding site of Ag-RDTs and theoretically alter the performance of Ag-RDTs in detecting the variant, the authors noted.

“The urgent need to reassess the performance of Ag-RDTs in detecting the SARS-CoV-2 Omicron variant is further compounded by early reports that Ag-RDTs have lower sensitivity for the Omicron variant than for other variants,” they added. Their study did not bear that out.

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