These Apps only really work when a large part of the population uses them. It can only record a contact if both persons are using the App. This means that conta

These Apps only really work when a large part of the population uses them. It can only record a contact if both persons are using the App. This means that contact detection frequency is more or less quadratic to the adoption frequency. If 1/2 of the population uses the app, you will only detect 1/4 of contacts. Likewise, with an adoption rate of 1/100 you will detect only 1/10000 of the contacts. Also there are a lot of technical problems with proximity detection. There is a wide range of hardware (especially on Android) which makes it very difficult to calibrate the App to give uniform results. You can get wrong positives, e.g. through a wall, and false negatives, e.g. due to shielding. It is a pity, the underlying algorithms are quite clever.