Credit: Hackney Council/Open Government Licence v3.0

Senior officials have said it is “not possible” to know how many people have enabled the NHS Covid-19 contact-tracing app.

Since its launch across England and Wales 13 months ago, the software has been downloaded almost 28.5 million times. But data has previously suggested that a significant proportion of users have not enabled the program’s contact-tracing capability and other key functions.

In April – when the app had been downloaded a cumulative total of 23.3 million times – government data reported that only 16 million users “had the app fully or partially enabled on their phone”. And even this reduced number included “users who have opted to disable contact tracing because the… on/off toggle in the app is a feature designed to enable users to turn off contact tracing in line with guidance, such as when storing their phone in a locker”.

Since then, all signs point to even more widespread disabling or deletion of the app; as cases have remained steady or risen, the number of contact-tracing alerts issued each week has declined steadily. 

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Over the summer, parliament’s Public Accounts Committee asked Dr Jenny Harries, the chief executive of the recently created UK Health Security Agency, to provide updated figures on how many people had enabled the app.

Her response, jointly sent with Department of Health and Social Care permanent secretary Sir Chris Wormald, indicated that the government now thinks it cannot produce a reliable estimate.

“There is no single definition or established measure of a user who has enabled the app,” said the letter, dated 4 August but only published by the committee yesterday. “Some people may pause contact tracing temporarily as there are several situations in which we advise pausing contact tracing temporarily, including if the user is a healthcare worker working in a healthcare building such as a hospital, or if someone works behind a Perspex screen and is fully protected from others. Due to these limitations and privacy-preserving design of the app, it is not possible to make a reliable and consistent calculation of the number of people with the app enabled at any one time.”

During the week in which almost all remaining restrictions on businesses and citizens were lifted, the app alerted 679,445 people in England that they had been in close contact with someone who then tested positive for coronavirus. This came in a week when 307,758 positive tests were recorded across the country.

The number of weekly exposure notifications declined sharply from that point and, for the last month or so, has plateaued around 120,000 to 130,000.

The most recent stats, for the seven-day period ending 13 October, shows that 132,036 exposure notifications were sent by the app, while 257,196 people tested positive.