Covid-19 and the false positive trap

Sep 17, 2020

Imagine a world where Covid-19 has been eliminated. To be certain this is true, the government conducts regular tests at random. The number of positive results should be zero, right? Wrong. There will always be a proportion of cases tested that come back with a false positive test result. Thankfully, for Covid-19, the false positive rate is less than one per cent of tests done. But it is not zero. It will be impossible for us to ever reach zero. Why? Because Covid-19 cannot be eliminated, even if it is likely to evolve to be more benign and become a seasonal problem like influenza.

Actual Covid cases during the epidemic were disproportionately seen among older people; in fact, 60 per cent were over 60 years old. Of the cases we are seeing now, only 11 per cent are in the over 60s, despite that age making up 24 per cent of the whole population. This means they are slightly under represented, but this may be a reflection of their willingness to be tested compared with younger people, and it would help to know the age of those tested for comparison. In contrast, only two per cent of cases were seen in the under 20s during the epidemic and this is now up to 19 per cent, much closer to the 24 per cent that would be seen if positive tests were distributed entirely randomly through the population.

A new case in an area that has not had a true case for months is extraordinary, as is any child ever testing positive for Covid. Extraordinary claims require exceptional evidence. Without genome sequencing, viral culture or chest CT results then any claim that a new Covid-positive case has suddenly appeared in an area or population must be questioned. Only by adopting this approach can we tackle the Covid false positive test problem.


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