The “world-beating” NHS Covid-19 app was systematically failing to tell people to self-isolate for more than a month, it’s emerged.
Unknown numbers of its 19million users were not ordered to quarantine despite contact with an infected person.
The software bungle is the latest hurdle to hit the app, which SAGE had warned has only a “marginal impact on transmission”.
Officials have no idea how many people were affected, but the Sunday Times reported it would be in the thousands.
The bungle involved the “risk threshold” in the app, which launched four months behind schedule in September.
When the app detects a Covid sufferer has had contact with another person, it uses the risk threshold to decide how infectious they were at the time. This is highest on the day someone develops symptoms, for example.
That score is then fed into an algorithm to work out whether their friend or family member needs to isolate.
However, officials have admitted the overall risk threshold was set five times higher than intended when the app launched.
The threshold was due to be lowered from 900 to 180, but this did not happen. Instead, it was only changed through an update last week.
That meant many people who would have been told to isolate under the new threshold were not told to isolate at the time.
That includes people who were sent ‘ghost’ notifications, saying they had contact with a Covid-19 sufferer but not to worry about isolating.
A government source told the Sunday Times “shockingly low” numbers of users had been sent full-blown orders to self-isolate.
App directors Randeep Sidhu and Gaby Appleton revealed the bungle in a blog post on the government’s website.
They wrote: “The ‘risk threshold’ was due to be lowered, but this change did not take place at that time.
“The updated version of the app addresses this by lowering the threshold at which users are deemed to be at risk of having caught the virus and alerted to self-isolate.
“The threshold was due to move from 900 to 180, but because we have a new statistical algorithm taking advantage of improved distance estimation, we are now lowering it to 120.
“The update to the risk threshold is expected to increase the number of people asked to self-isolate by the app, having been in close contact with someone who has tested positive.
“We believe lowering the threshold is necessary to reduce the R rate and break the chain of transmission.”
Government officials said it was not possible to retrospectively calculate how many people would have been told to isolate.
They said there was no right or wrong setting for the risk threshold - instead it was a “judgement call” made by human beings.
A Department for Health and Social Care spokeswoman said: “The NHS Covid-19 app is the only app in the world using the latest Google Apple technology to better gauge distance to identify those most at risk, and is deemed ‘excellent’ by international standards.
“As previously published, we anticipate more app users who are at high risk of having caught the virus will receive a notification to self isolate, and that will be to everyone’s long term benefit by reducing the chances of those with the virus passing it onto others.”
