Shoppers pass by a sign promoting the NHS COVID-19 app | Photo by PAUL ELLIS/AFP via Getty Images
What happens when an app designed to help people navigate the real world doesn’t match up with reality? That’s what happened recently with a National Health Service contact tracing app in England and Wales, which failed to notify many users that they had been exposed to the virus that causes COVID-19.
Like many contact tracing programs, the NHS app keeps track of when a user is close to other people who also downloaded the app. If someone tests positive, the app can send an alert to all their contacts, particularly the ones that they’ve been around the most. The app calculates a person’s risk based on a few factors, including how long they’ve been close to the person who tested positive. If the calculation passes a certain threshold, an…