We don’t know how many people in the UK have Covid-19 and the symptom tracker app doesn’t tell us

27th Mar 2020

Claim

Data from a new app which tracks symptoms of Covid-19 claims that up to 6.6 million people in the UK have the disease.

Conclusion

This data cannot be extrapolated to the UK population. The number of people with Covid-19 may be higher, lower, or the same as claimed.

“Coronavirus symptom tracker app suggests 6.6MILLION Brits already have Covid-19” 

The Sun, 26 March 2020

“Coronavirus symptom tracker claims up to 6.6MILLION people in Britain may already have the life-threatening infection” 

Mail Online, 26 March 2020

“Data from the free Covid Symptom Tracker suggests that one in 10 Britons may now be carrying the virus.” 

Daily Telegraph print edition, 26 March 2020

Several newspapers reported findings from King’s College London that 10% of the 650,000 people who have signed up to its app to track users’ Covid-19 symptoms, are currently showing symptoms. 

The Telegraph, Mail and Sun extrapolated these findings to suggest that, if that 10% figure was reliable, then as many as 10% of the UK (around 6.6 million people) could have Covid-19 currently. 

We don’t currently know what percentage of the UK population may have been infected with the new coronavirus, but a simple calculation like this doesn’t get us closer to knowing. There are two main problems with extrapolating in this way.  

The first is that the 650,000 people on this app may not be representative of the UK population as a whole. For example, you might expect people with symptoms to be more interested in signing up in the first place compared to people who don’t. A press release from King’s College (published after the news articles above were written) said that young people are far more likely to use the app than older people. 

As there’s no evidence that the app’s users accurately represent the general population, it’s wrong to extrapolate the findings from the app up to the UK population level.

The second main problem is that even if users were representative of the UK population, having symptoms of Covid-19 doesn’t mean that someone has the disease. 

The main symptoms of Covid-19 are a high fever and a persistent cough. These are also symptoms of lots of other diseases.

And, on the flipside, not having symptoms doesn’t necessarily mean that someone isn’t infected with the new coronavirus. 

There have been various reports on what proportion of people with Covid-19 do not show symptoms. At this stage it’s still not known how common this is, but one Italian academic found that between 50% and 75% of residents who tested positive in one town in Veneto, Italy did not show symptoms. 

Some of these issues are covered in the reporting, but extrapolating the figures in the first place goes way beyond what the evidence supports.