Smartphone displaying contact tracing app.

During the first wave of COVID-19, researchers at Oxford University built a computer model that suggested if 56% of the UK downloaded and used a contact-tracing app (alongside other control measures) it could end the epidemic in the country.

With the English app only available since September, it’s too early to tell how the system is actually doing. But even based on other countries whose apps have been available much longer, there’s still very little evidence that they can make a real difference to fighting COVID-19 – or that they can’t.

While this doesn’t mean we should write off contact-tracing apps altogether, the lack of evidence is a concern given the focus and money devoted to these apps and the policy decisions made around them. This kind of “tech solutionism” could be a distraction from developing proven manual contact-tracing systems. Indeed, the Council of Europe has raised the question of whether, given the lack of evidence, the promises made about these apps are “worth the predictable societal and legal risks.”

Despite predictions that between 67.5% and 85.5% of potential app users would download apps, worldwide download rates of contact-tracing apps have so far been low, running approximately at 20%. In Germany, it has been around 21%, in Italy 14%, in France just 3%. Iceland and Singapore, which was one of the first to launch an app, have the highest download rates to date at 40%.

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Download rates matter because you need lots of other people to have the app on their phone to increase the chance that if you come into contact with someone who has the virus the system will be able to alert you of that fact.

In broad terms, if you have 20% of the population as active app users then there is only a 4% chance of coming into contact with another app user (the maths is explained here). Increase the download rate to 40% and you have a 16% chance of meeting another active app user. This also works on the assumption that users have the same app or different ones that can work together.