RE: Positive / negative COVID tests queried on BBC5 Sep 2020 10:03
Force, you raise an interesting question and there has been a lot of research in other viruses. Firstly, the term ‘dead’ virus is an anathema. Viruses are not alive. They are beautifully minimalist non-living structures that employ a living cell to replicate. They have a very simple set of instructions for the protein container (viral capsid) and nucleic acids (RNA (sometimes DNA). Any viral genome whether it is protected in a capsid or not will be infectious if it is intact in a host cell. Viral RNA outside of a cell and not in a viral capsid is unlikely to be infectious unless it enters a cell – note the word unlikely, research goes on since most of these things come down to probability.
All PCR based tests detect either DNA or with RT-PCR, RNA. It cannot distinguish between viral RNA that’s in a cell and capable of generating new virus or not. But the bottom line is it will be the minority of patients who have cleared the virus that are walking around with naked viral RNA outside of cells for long periods of time because RNA is very fragile. It is far safer to wait to clear all residual viral RNA. The question then becomes how long that timeframe is.
From a technical PCR perspective, the number of cycles is an indication of 'how much' RNA was present in the sample. It is wrong to assume that if only a little RNA is present that this correlates to RNA that was not inside viral capsids. There simply is no link.
You might look at setting thresholds for each of the two viral targets used in the test on the assumption that naked viral RNA would not be intact and would become fragments so that it would provide differential results for the two targets. That would be highly speculative and there certainly would not be enough epidemiological data to support either that RNA from outside the virus is persistent, non-viable if it re-entered a cell, or becomes fragmented.
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