Officials estimate that it is 70pc accurate for “positive” matches – that two people definitely came within two metres of each other.
However, it might also overcompensate, with a 45pc “false negative” rate, meaning people may be matched up when they were in fact further away.
A range of experts continue to assert the contact tracing app could be a red herring, even as the number of human contact tracers is reduced by 6,000.
Prof Ross Anderson, a computer science expert at the University of Cambridge, is scathing: “What works is human test and trace. Lots of countries have rushed to do apps but have given them up as not being very helpful.”
The prevalence of false negatives could be a sticking point.
Anderson adds: “People are not going to run an app that as a result of a false negative is telling them to stay at home.”
If anything, the new plans for the UK’s contact tracing app seem more complicated than before.
The app will anonymise Bluetooth contact tracing, as was originally planned.
But it will also be able to scan QR codes for people entering buildings, give feedback on a user’s “risk score” and let them book a test.
One view is that the original app was not likely to hook users and NHSX had focused too much on privacy.
“It has all been pulled away from NHSX,” says one source. “The people who are running it get it. They understand user engagement.”
For those developing the app, adding extra functionality is seen as critical to building and maintaining uptake. Around 50pc to 60pc of the population will be required to download the app for it to be truly effective.
The thinking is that an added focus on QR codes and local risk could attract more users than a “boring” app, as one source close to the project puts it.
Thompson, the app’s new boss, says: “It’s like NHS Test and Trace in your pocket.”
Such an approach seems to run counter to the minimalist, privacy-first aims that have been mandated by Apple and Google, although it is understood the Government has worked closely with the tech giants.
According to Dr Eddie O’Neill, who has been working on Northern Ireland’s system, their philosophy is “the app fills the gap”, finding minority cases that regular contact tracing misses. By contrast, England’s approach may raise fears, again, of mission creep.
Imogen Parker, of AI research group the Ada Lovelace Institute, says: “There is a question over bolting on extras. Is it a solution looking for a problem? That could be a red flag.”