Highly sensitive nucleic acid amplification tests (NAATs) designed to detect SARS-CoV-2 RNA are the standard of care for the diagnosis of COVID-19. However, the accuracy of these methods for the quantitation of active virus rather than non-infectious RNA fragments that can persist for extended periods of time has been unclear. This issue is particularly relevant for congregate care patients who are unable to return to their home residence until fully negative by NAATs. We tested paired samples from individual patients for the presence of virus at both early and later stages of disease. Culture of nasopharyngeal swab samples for 10 days in Vero E6 cells revealed active virus in only 4 out of 14 (28.6%) patients. The ability to isolate viral plaque-forming units (PFU) correlated with viral RNA loads of >6.79 log genomic copies/ml and only occurred in samples collected from patients early after symptom onset and before development of antibody. Culture in Vero E6 cells lacking the STAT1-dependent interferon signaling pathway increased the numbers of viral PFU detected but did not affect the incidence of positive cultures. We conclude that culturable virus is correlated with SARS-CoV-2 NAATs detection only during early symptom onset and with high viral titers/low antibody titers in non-immunosuppressed patients.
Keywords: Plaque assay; RT-qPCR; SARS-CoV-2; Sub-genomic RNA; Vero STAT1(−/−).
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