Intracellular flow cytometry complements RT-qPCR detection of circulating SARS-CoV-2 variants of concern

Basic and antiviral research on SARS-CoV-2 rely on cellular assays of virus replication in vitro. In addition, accurate detection of virus-infected cells and released virus particles is needed to study virus replication and to profile new candidate antiviral drugs. Here, by flow cytometry, we detect SARS-CoV-2 infection at single cell level and distinguish infected Vero E6 cells from uninfected bystander cells. Furthermore, based on the viral nucleocapsid expression, subpopulations of infected cells that are in an early or late phase of viral replication can be differentiated. Importantly, this flow cytometric technique complements our duplex RT-qPCR detection of viral E and N, and it can be applied to all current SARS-CoV-2 variants of concern, including the highly mutated Omicron variant. Method summary This study describes the characterization of SARS-CoV-2 infected cells using intracellular flow cytometric viral nucleocapsid staining that complements RT-qPCR quantification of viral RNA. The technique makes it possible to distinguish between infected cells in the early (low N) or late phase (high N) of viral replication. It can also be applied to the different variants of concern of SARS-CoV-2, including the Omicron variant.