Britain's health ministry on Thursday launched its delayed contact tracing app for England with fresh trials after technical issues prompted a rethink of its approach and a change of system.
The test and trace programme is key to detecting flare-ups of Covid-19 and reopening the economy but has been dogged by problems.
A smartphone app developed by the National Health Service (NHS) was initially expected to be rolled out in May but did not materialise, and in June, the government pivoted away from a homegrown model to a system developed by Apple and Google .
"We've worked with tech companies, international partners, privacy and medical experts to develop an app that is simple to use, secure and will help keep the country safe," health minister Matt Hancock said.
The app will be tested on the Isle of Wight to the south of England, as the first app was, as well as with healthcare workers.
It will log how long and how close a person has spent near another and alert them if they later test positive for Covid-19, though the ministry said it was designed with privacy in mind "so it tracks the virus, not people".
It will also let people scan barcode-like QR codes to log venue visits.
The privacy-centric, decentralised system of the Apple-Google model contrasts with the centralised approach Britain had used before, and the app's launch follows a pivot towards a more local approach for test and trace.
On Monday, the government said the tracing scheme would become more locally targetted, and reduce the number of national-level contact tracers.
Despite the delays and problems with the test and trace system, Prime Minister Boris Johnson has described it as "world beating", while officials have played down the centrality of the app, saying it is the "cherry on the cake" of the programme.