So a cool thing that Germany did early on was they released an app that collects step and pulse data from your fitness tracker in order to try to find outbreaks. (I've been using the app from the start). They recently released an analysis and... it seems to be working

If the average resting heart rate increases and the number of steps decreases, that's apparently a strong diagnostic sign of fever. And with one important caveat*, it's actually done a decent job. It saw the second wave coming before it appeared in test results

(The caveat is that temperature in particular affects pulse, which is probably why the number of fevers detected in summer increased a bit while infections were flat.) Anyway, you can read more about it (in German) here: https://corona-datenspende.de/science/reports/improved-method/

Oh wait, or you can read about in English here! https://corona-datenspende.de/science/en/reports/improved-method/

Side note - here's a map of the average pulse across Germany (in late spring/early summer). Amazing what you can do with technology these days https://corona-datenspende.de/science/en/reports/pulse/

(oh and the other caveat I forgot to mention: of course, not all fevers are caused by COVID. It also seems to pick up flu etc)