Track Omicron's spread with molecular data

On 26 November, the newly emerged variant Omicron was designated a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant of concern (VOC) (1). Rapid polymerase chain reaction (PCR) test results could improve estimates of the prevalence of Omicron around the world. The widely used Thermo Fisher TaqPath COVID-19 PCR assay was valuable in tracking the spread of the Alpha (B.1.1.7) VOC (2) because a deletion of amino acids 69 and 70 in Alpha’s spike gene (Δ69–70) yields a distinct absent S-gene (S−) despite positive test results. The Delta VOC lacks this deletion and is therefore S-gene positive (S+) on TaqPath PCR tests (3). The Omicron VOC shares the spike Δ69–70 deletion with Alpha, which has dropped to negligible levels worldwide. Therefore, the frequency of S− results can be used as a rapid proxy for the frequency of Omicron cases, provided initial detection of local circulation had been confirmed by sequencing.