I've been pondering over the idea of offering the option to use physical device, like e.g. a Bluetooth bracelet, for contact tracing in addition to apps.
For contact tracing to have an impact at all, we need a quite large percentage of the population to use one of these apps.
Even if 60% of the population had some kind of app installed and this app worked properly, we would still only detect just 36% of all new infections, since both parties (infected person and person to be infected) need to use the app.
There is a significant portion of the population that does not want to or cannot use such an app, e.g. the elderly, kids too young to have their own smartphone, people with certain disabilities, people that can't carry their phone with them all the time (e.g. while doing sports / working) etc.
This population can still be relevant in spreading the virus - for instance, when loosening lockdowns, young children attending kindergarten / school can bridge the gap between families.
Moreover, even among those that own a smartphone and that want to use the app, I just can't see it all work flawlessly.
Outside of the tech bubble, I see many people with older Android / iOS versions that don't receive updates (which might be crucial for contact tracing to function without having to keep the app open at all times) or people simply failing to install updates.
We also don't want the app to be too sensitive (an infected person that happens to be at the opposite end of the same subway car shouldn't trigger quarantine for you), but also not too insensitive (people might put their phone in handbags or attenuate BLE radio waves with their body).
I think that these problems could be solved by offering something like a standalone Bluetooth bracelet, compatible with whatever App becomes the standard.
It should be possible to mass-produce these relatively cheaply (The only technical problem I see is that the physical bracelet would need to receive a (trusted) "list of infected IDs" somehow.
Maybe a mesh network of bracelets with smartphones as information providers could work? Maybe bracelets could connect to public WiFi? Or maybe we could leverage some existing low-tech data broadcast infrastructure such as RBDS/RDS (Radio Data System)?