Optimisation of SARS-CoV-2 culture from clinical samples for clinical trial applications

Clinical trials of SARS-CoV-2 therapeutics often include virological secondary endpoints to compare viral clearance and viral load reduction between treatment and placebo arms. This is typically achieved using RT-qPCR, which cannot differentiate replicant competent virus from non-viable virus or free RNA, limiting its utility as an endpoint. Culture based methods for SARS-CoV-2 exist; however, these are often insensitive and poorly standardised for use as clinical trial endpoints. We report optimisation of a culture-based approach evaluating three cell lines, three detection methods, and key culture parameters. We show that Vero-ACE2-TMPRSS2 (VAT) cells in combination with RT-qPCR of culture supernatants from the first passage provides the greatest overall detection of Delta viral replication (22/32, 68.8%), being able to identify viable virus in 83.3% (20/24) of clinical samples with initial Ct values <30. Likewise, we demonstrate that RT-qPCR using culture supernatants from the first passage of Vero hSLAM cells provides the highest overall detection of Omicron viral replication (9/31, 29%), detecting live virus in 39.1% (9/23) of clinical samples with initial Ct values < 25. This assessment demonstrates that combining RT-qPCR with virological end point analysis has utility in clinical trials of therapeutics for SARS-CoV-2; however, techniques may require optimising based on dominant circulating strain.