Highly sensitive and specific detection of the SARS-CoV-2 Delta variant by double-mismatch allele-specific real time reverse transcription PCR

Background: The highly transmissible Delta variant of SARS-CoV-2 (B.1.617.2), first identified in India, is currently replacing pre-existing variants in many parts of the world. To help guide public health policies it is important to monitor efficiently its spread. Genome sequencing is the gold standard for identification of Delta, but is time-consuming, expensive, and unavailable in many regions.

Objective: To develop and evaluate a rapid, simple and inexpensive alternative to sequencing for Delta identification.

Methods: A double-mismatch allele-specific RT-PCR (DMAS-RT-PCR) was developed. The technique exploits allele-specific primers, targeting two spike gene mutations, L452R and T478K, within the same amplicon. The discriminatory power of each primer was enhanced by an additional mismatch located at the fourth nucleotide from the 3' end. Specificity was assessed by testing well characterised cell culture-derived viral isolates and clinical samples, most of which had previously been fully sequenced.

Results: In all cases the results of viral genotyping by DMAS-RT-PCR were entirely concordant with the results of sequencing, and the assay was shown to discriminate reliably between the Delta variant and other variants (Alpha and Beta), and 'wild-type' SARS-CoV-2. Influenza A and RSV were non-reactive in the assay. The sensitivity of DMAS-RT-PCR matched that of the diagnostic SARS-CoV-2 RT-qPCR screening assay. Several samples that could not be sequenced due to insufficient virus were successfully genotyped by DMAS-RT-PCR.

Conclusion: The method we describe would be simple to establish in any laboratory that can conduct PCR assays and should greatly facilitate monitoring of the spread of the Delta variant globally.