The COVID-19 exposure notification app used in England and Wales failed to warn users if they were in close contact with potentially infectious COVID-19 patients. Because of the error, thousands of people were not told to quarantine even when they should have been, according to the Sunday Times, which first reported the flaw.

The app was launched on September 24th, and there have been 19 million downloads since. It’s built using Google and Apple’s Bluetooth Low Energy-based system, which monitors nearby phones. If someone tests positive for COVID-19, their app can alert the phones that they were in contact with. The UK originally planned to use its own app, sidestepping the Google and Apple system, but reversed course in June.

Since its launch, the app flagged few users for possible exposure, according to the Sunday Times. Engineers figured out the reason why last week. The app was originally built to simply recommend that anyone closer than 2 meters for more than 15 minutes to someone who later tested positive should quarantine, according to The Guardian. But just before the launch, it was adjusted to take into account when the sick person’s symptoms began.

Research shows that people tend to have high levels of virus in their nose and throat, and may be more contagious, around a day before they start to show symptoms. Levels stay high the first few days of symptoms, and then drop off. If someone was in contact with a sick person outside of that window, the UK app would consider the interaction less risky. If they were in contact inside of that window, on the other hand, it would only take three minutes of contact to trigger an alert.

The adjusted app did calculate those new risk levels. But the thresholds at which a person would actually get an alert were left unchanged, according to a government blog post. Without the updated thresholds, a user could have spent up to 15 minutes with a highly infectious person and up to 40 minutes with a less-contagious person without getting an alert, according to The Guardian.

The error was discovered when the engineering team added another update, which could more accurately manage exposures at a distance over a meter away.

A UK Department for Health and Social Care spokesperson told The Guardian that the app was “the only app in the world using the latest Google-Apple technology to better gauge distance to identify those most at risk.”

https://www.theverge.com/2020/11/2/21546618/uk-coronavirus-contact-tracing-app-error-alert-isolation