Development of highly specific singleplex and multiplex real-time reverse transcription PCR assays for the identification of SARS-CoV-2 Omicron BA.1, BA.2 and Delta variants

The Omicron variant of SARS-CoV-2 (B.1.1.529), first identified during November 2021, is rapidly spreading throughout the world, replacing the previously dominant Delta variant. Omicron has a high number of mutations in the spike gene, some of which are associated with greatly increased transmissibility and immune evasion. The BA.1 sublineage has been most prevalent but there is recent evidence that the BA.2 sublineage is increasing in proportion in many countries. Genome sequencing is the gold standard for Omicron identification but is relatively slow, resource intensive, of limited capacity and often unavailable. We therefore developed a simple, rapid reverse transcription PCR (RT-PCR) method for sensitive and specific detection of the Omicron variant, including both the BA.1 and BA.2 sublineages. The assay targets a total of 5 nucleotide mutations in the receptor binding domain of the spike gene that give rise to 4 amino acid substitutions at G339D, S371L, S373P and S375F. The forward primer was designed as a double-mismatch allele specific primer (DMAS) with an additional artificial mismatch located four nucleotides from the 3 end to enhance binding specificity. Assay specificity was confirmed by testing a wide range of previously-sequenced culture-derived viral isolates and clinical samples including the Alpha, Beta and Delta variants and wild type SARS-CoV-2. Respiratory syncytial virus and influenza A were also tested. The assay can be run in singleplex format, or alternatively as a multiplex RT-PCR to enable Omicron and Delta variants to be detected and distinguished within the same reaction by means of probes labelled with different fluorescent dyes. Sublineages BA.1 and BA.2 can be differentiated if required. The methods presented here can readily be established in any PCR laboratory and should provide valuable support for epidemiologic surveillance of Omicron infections, particularly in those regions that lack extensive sequencing facilities.