A low-cost molecular test for SARS-CoV-2 detection suitable for variant discrimination and community testing using saliva

Abstract

The gold standard for COVID-19 diagnostic testing relies on RNA extraction from naso/oropharyngeal swab followed by amplification through RT-PCR with fluorogenic probes. While the test is extremely sensitive and specific, its high cost and the potential discomfort associated with specimen collection make it suboptimal for public health screening purposes. In this study, we developed an equally reliable, but cheaper and less invasive alternative test based on a one-step RT-PCR with the DNA-intercalating dye SYBR Green, which enables the detection of SARS-CoV-2 directly from saliva samples or RNA isolated from nasopharyngeal swabs. Importantly, we found that this type of testing can be fine-tuned to discriminate SARS-CoV-2 variants of concern. The saliva RT-PCR SYBR Green test was successfully used in a mass-screening initiative targeting nearly 4500 asymptomatic children under the age of 12. Testing was performed at a reasonable cost of less than € 0.8 per child, and in some cases, the saliva test outperformed nasopharyngeal rapid antigen tests in identifying infected children. Whole genome sequencing revealed that the antigen testing failure could not be attributed to a specific lineage of SARS-CoV-2. To further reduce testing costs, we produced all the necessary enzymes and established a new RT-PCR protocol based on the EvaGreen dye. Overall, this work strongly supports the view that RT-PCR saliva tests based on DNA-intercalating dyes represent a powerful strategy for community screening of SARS-CoV-2. The tests can be easily applied to other infectious agents and, therefore, constitute a powerful resource for an effective response to future pandemics.