Covid-19: NHS app has told 1.7 million to self-isolate By Leo Kelion image captionApproaching two million people have been told to self-isolate by the app The NHS Covid-19 app has told 1.7 million people in England and Wales to self-isolate to date. Health ministers have also revealed they believe it has prevented about 600,000 cases of the disease. In a further disclosure, internal data indicates that about 16.5 million people are currently actively using its contact-tracing tool. That figure is 24% below the app's latest download tally, which is the government's preferred measure. The discrepancy is likely to be down to people uninstalling the app, turning off its contact-tracing capabilities, or simply failing to have activated it in the first place.