A reported error with the NHS Covid-19 App has left potentially thousands of people unaware they may have been exposed to coronavirus.

The contact-tracing app was set at the wrong sensitivity level, meaning many users were not sent self-isolation alerts after they came into contact with infected people.

The error meant users whose “risk score” should have triggered an alert were not notified, The Sunday Times first reported.

The app, launched a month ago on 24 September, has been downloaded more than 19 million times. It was updated last week to improve accuracy and notifications which was “expected to increase the number of people asked to self-isolate by the app”.

Since its launch “shockingly low” numbers of people had been sent warnings about potential exposure to the virus, a government official told The Sunday Times.

The app uses Bluetooth to track time and distance between devices to determine a user’s risk. The technology should have been recognising if people had been in close enough proximity to be at risk of the virus, but was instead recording them as being too far away.

Last weeks update saw the risk threshold lowered to improve the apps accuracy and notifications, meaning more people would be notified if they had been exposed.

But the updates had been available before the apps launch in September, developers admitted in a blog post. It means the app could have been upgraded to be more accurate from the tested version in the Isle of Wight and London borough of Newham.

To accommodate a persons infectiousness, which is highest on the day they develop symptoms, the risk threshold was “due to be lowered” however “this change did not take place at that time” wrote Randeep Sidhu and Gaby Appleton, who are leading figures behind the app.

A Department of Health and Social Care spokesperson 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.”

It is not known how many people have been told to self-isolate through the app.

A bumpy road

The app has been plagued by problems since it was first reported NHSX were developing one in March.

Privacy concerns about the centralised approach originally taken saw the first version of the app, initially trialled in May, scrapped in June.

NHS Test and Trace, which by then had taken over the development of the app, confirmed it would instead be using Apple and Google’s APIs which were based on a decentralised approach – meaning data only ever passes between devices.

New trials of the app were launched in August before the app was officially launched in England and Wales in September – the last countries in the UK to have an app available.

But within days of its launch an urgent fix was needed when it was revealed thousands of test results could not be linked to the app.

Further updates were made last month to fix phantom messages telling people they had possible Covid-19 exposure that would disappear when the notification was opened.

Users said the messages were worrying and caused confusion over whether they had been exposed and needed to isolate or get a test.

Last week’s updates aimed to remove these notifications. A DHSC spokesperson confirmed official notifications from the app won’t disappear when a user clicks on them and will provide appropriate advice.