It’s not often I dare to disagree with Ross Anderson, but most of these objections don’t make a lot of sense to me. There’s nothing to stop the app from phoning home once a contact is identified, so all of the human-in-the-loop stuff is still possible.

The only difference between a centralised and decentralised solution so far as I can tell is whether or not we end up with massive centralised databases of every social contact between people who are not infected. Collecting that data seems completely unjustifiable to me, given that it is by definition unnecessary.