False positives are ok, even a massive amount of false positives. Currently we all have to change our behaviour around everyone, all the time. Even if you are

False positives are ok, even a massive amount of false positives. Currently we all have to change our behaviour around everyone, all the time. Even if you are in New York that probably means there are “false positive” behaviour changes (behaviour changes for no reason) all the time – simply because we lost control of the situation and cannot currently contact-trace our way out of that situation. The idea behind these contact tracing apps is that they make only really sense once you have to outbreak well under control (through drastic measures). The goal is then to relax measures (probably not to abandon all of them – handshaking is probably dead and the recommendation will remain to stay away from other people in public and forget large gatherings for a long time, as well as a lot of travel …) and be much more targeted with who you impact through such contact tracing. Obviously they cannot replace proper contact tracing, but they can be a helpful tool in the tool-belt and increase the effectiveness of standard contact tracing and testing. If there are, say, 500 detected active cases in a country like Germany (which is probably around the limit of where contact tracing still works, even with many more people working contact tracing and much expanded testing capability) contact tracing apps can provide an additional signal and increase the speed of standard contact tracing approaches, especially since we know that much of the spread is pre-symptomatic. Even if this tool overestimates the infectious contacts someone had hundredfold and we assume a ridiculous R0 like 10 (so someone infects ten people on average but a hundred times that many people are actually detected by contact tracing – so 1,000 people altogether) that would still only place at most 500,000 people in quarantine (assuming an upper limit of 500 detected active cases that can be handled before you lose control again and have to place the whole country under lockdown). That’s a huge number but probably not as impactful at all as the current measures that are in place. (Obviously you need to put measures in place to make that possible: mandatory, automatic and paid sick leave for anyone who is told to stay at home for a week or symptom-end plus one week. Stuff like that. Some countries are probably better prepared for that than others.) Even this blunt instrument would be much better than the current situation.