October 30, 2020 - 7:00am

A strange little conspiracy theory is going around in parts of the internet. They’ve noticed that flu cases are way down — a September paper from the US CDC reckons by 90%, while this piece makes an (unsourced) claim of 98%. And they have decided that this is because flu cases are being misdiagnosed as Covid. The ur-conspiracist David Icke himself is on board with this one.

To be clear: it’s not true. It is inconsistent with how flu and Covid cases are counted, and there is a much simpler explanation, which is that the measures we’ve taken to counter Covid are also effective against other respiratory diseases. But it has got quite a few people talking about it, and some of the tweets about it are doing fairly big numbers.

Here’s why it’s not true. First, the prevalence of Covid — in the UK, at least — is estimated using tests which look for the presence of RNA from the SARS-Cov2 virus. For instance, on Thursday the REACT-1 survey estimated that around 1.3% of the UK population has the virus. It does this by sampling the population at random, as in an opinion poll, and extrapolating from that. The ONS infection survey does much the same thing.

Similarly, flu prevalence is estimated by taking laboratory-confirmed cases and extrapolating from that to provide an estimate of the total in the population. The two are entirely different kinds of virus — SARS-Cov2 is a coronavirus, flu an influenzavirus — and there just isn’t a way that a test for one could be triggered by the presence of the other. The CDC estimate, mentioned in the first paragraph, is based on positive laboratory tests. That would not be affected by how many Covid tests had come back positive.

The CDC paper says that early on, people thought it might have been because fewer flu tests were being done — due to people with respiratory illnesses being sent for Covid tests — but “renewed efforts by public health officials and clinicians to test samples for influenza resulted in adequate numbers tested and detection of little to no influenza virus”.

Of course, that doesn’t explain why the flu cases appear to have gone down so much. But that seems fantastically non-mysterious to me: the two diseases are spread by similar means, so the measures we take to slow the spread of Covid-19 (masks, lockdowns, travel restrictions, etc) will also slow the spread of flu.

Flu is less easily transmissible than Covid-19 — I’ve just looked at a bunch of studies and they all broadly agree with this one which estimates that a new flu strain has an R of between 1.4 and 1.6, compared to about 2.6 for Covid. If wearing face masks reduces R by 0.7, doing that on its own would only slow the spread of Covid while it would cause flu to (eventually) die away. It could easily be that the measures we’ve taken haven’t been enough to stop Covid but have had a huge impact on the flu season.

There may be other things going on, but I think (as does the CDC) that is the likeliest explanation. It’s just not plausible that people are counting flu cases as Covid.

Tom Chivers is a science writer. His second book, How to Read Numbers, is out now.