State-of-the-art of virological diagnosis of SARS-CoV-2 infection

The COVID-19 infection has emerged suddenly in China, in December 2019, and spread rapidly worldwide. The virus responsible for this dramatic outbreak was soon identified as a new coronavirus derived from a bat virus that was named SARS-CoV-2, which allowed the design of molecular tests able to detect the viral RNA in clinical specimens. In addition to a few data on this new virus, the aim of this review is to present the molecular tests that are used for the direct detection of the viral genome and the serological tests that are available to characterize this infection. The diagnosis lays mainly on the recognition of the viral RNA by real-time RT-PCR in the respiratory secretions (naso-pharyngeal specimens at the early phase of infection and deep respiratory specimens at the stage of pneumonia); the results are available in approximately 4 hours of time. The peak of infectivity occurs between 3 days before and 3 days after the onset of symptoms. The viral RNA can also be detected in blood and stools but the infectivity of these samples has not been demonstrated yet. At a later stage of infection, an anti-SARS-CoV-2 humoral answer can be recorded, with IgM and IgA specific antibodies as soon as 8 or 9 days after the onset of symptoms; later, IgG specific antibodies sign an earlier contact with the virus. The detection of IgG antibodies may be delayed, notably in subjects who presented a mild or asymptomatic infection. Many unsolved questions are still pending, notably concerning the protective efficacy of this humoral answer and its duration, together with its role in the pathophysiology of severe infections.