I just had to self report a positive test on the SlowCOVIDNC app. It’s a contact tracing app that goes back 14 days from the reported positive test and calculates a risk factor for all mobile phones using the app that were within Bluetooth range of the reporter’s phone. If the risk factor exceeds a threshold there’s a push notification to the user of the phone to recommend a test.
It’s funny, there’s no real sense of closure to it. With your positive test they assign you a 10 digit number that you voluntarily enter into the app and that’s it. They don’t tell you if you were close enough to anyone in the last two weeks to trigger a notification, although I must have been at least close enough for one transmission unless I got it from surface contamination, which is possible I guess.
I guess a second question would be; Has anyone had a contact tracing app recommend a test? The problem with these things are that if people don’t use them they can’t really help, so my reporting is in all likelihood pointless, but it was not a great deal of trouble, so it doesn’t have to be a great deal of benefit.
That’s intentional with the apps that are based on the GAEN (Google/Apple Exposure Notification) API and I believe that SlowCOVIDNC is one of them. The servers know how many positive tests are entered (but not who the people are), but don’t have a way of determining how many people are affected by those. Your phone keeps the necessary ledger of temporary IDs that have met the necessary criteria to be "at risk of transmission"and then occasionally asks the server for a list of new positive IDs since the last check-in. The app on your phone then determines if an entry on the list matches one on your phone and notifies you if it does.
As for effectiveness, since the app servers can’t really see who is notified, many locations have done surveys or studies to try to gauge the effectiveness, and some pretty significant numbers are being touted. While the numbers should be taken with a grain of salt, since we do know that a massive number of positive tests have been recorded, we can at least assume that some number of people are being notified to get tested.