The SARS-CoV-2 transmission riddle - Part 3

Binary PCR positivity is a meaningless test result on its own

While immersed in reviewing studies, we soon realised that a “case” meant different things depending on where you looked.  In September 2020, we documented these differences. We were confused then, as much as now, by one central question: when is Covid-19 Covid-19?

Most if not all confirmed cases are based on a positive polymerase chain reaction (PCR) test. This molecular test looks for tiny viral fragments from one or more samples from the same person. It cannot tell the difference between a virus fragment or the whole virus.

In the absence of external contamination, PCR is very “sensitive”.  It will identify the pre-specified viral fragments you are looking for, no matter how much in the sample. 

But here’s the rub: PCR positivity was equated with the only type of case that matters in an epidemic: the infectious case, which can pass the infection on.  This point is central to our thinking and everything we discuss from here on.  What comes next won’t make sense if you don't get this.

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