What is the time required for the body to produce a detectable concentration of anti SARS-CoV-2 ? Would such lag time make serological tests less interesting from a screening perspective?
Should focus be shed on RNA/other viral component based tests that also address the issue of time/cost of PCR based tests?
It depends on the question you want to answer. For fast detection of acute infection, PCR seems favorable over antibody detection. Serology may be negative in the first days after onset of symptoms while the viral RNA is already detectable at that time. In a current publication by To and colleagues in Lancet Infectious Diseases, IgM and IgG were shown to become detectable 5 – 10 days after onset of symptoms (https://doi.org/10.1016/S1473-3099(20)30196-1). Moreover, in some cases IgG was detectable earlier than IgM, which means that IgM is not useful as marker of early disease. As such, when aiming to detect infected individuals early to disrupt infectious chains, serology is therefore not helpful.
Serological (mass) screening means that the actual status of the screened individuals is unknown. This means that detection of IgG just allows the conclusion, that the positive individuals probably have had contact with the virus. The time of infection remains unknown (could be days, weeks, or even month ago). Given, that IgG are indeed protective, positive IgG serology allows for stratification of who is allowed of going back to work.
With IgM, the situation is more complex, since IgM has usually less specificity than IgG. We know from other viral diseases, that IgM can increase non-specifically (e.g. CMV-IgM in case of EBV infection). What do we actually know about the specificity of anti-SARS-CoV-2 IgG, IgM, IgA? What do we know about their protective potential?
Serological mass screening will most likely come, but the actual value remains unclear, as long as sensitivity and specificity of the tests/antibodies are not determined and the protective character of the antibodies has been demonstrated.
May I ask a question, how long can you actually detect the viral RNA via PCR ? Is it even possible to detect the virus via PCR after lets say 7 days of infection ?
Jan-Frederik Wiesenberg According to reports in the literature, viral RNA can be detected up to 20 days after onset of infection. To an coworkers (TO, Kelvin Kai-Wang, et al. Temporal profiles of viral load in posterior oropharyngeal saliva samples and serum antibody responses during infection by SARS-CoV-2: an observational cohort study. The Lancet Infectious Diseases, 2020) reported on one patient, who was tested positive until 25 days, following occurrence of the first symptoms. It's just about RNA detection, no clue as to whether infectious virus are still spread after that time.
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