In that Nature paper, a key number leaps out. The SAR – the secondary attack rate – is the percentage of those sent into isolation after a close contact with someone infected, who then go on to test positive for the virus during their isolation period. Last autumn it was 6% for the app – which sounds low, but is roughly comparable to the rate for those phoned up and told to isolate by the manual, human-operated Test and Trace operation.

https://www.bbc.co.uk/news/technology-57836325