StrawberryJam imo, it should be tests that should drive tracing,
since these will have the highest chance of producing more positive contacts
The system in Germany from February has been that as soon as someone receives a positive test,
they are contacted by tracking teams based in the local authority.
There were > 20,000 trained trackers, possibly more now
Contacts are traced and then tested themselves
If there are sufficient positive tests in a locality
- 50 / 100,000 population within 7 days -
then there is a localised lockdown around the epicentre e.g. a meat plant
From the start, anyone found positive was given a formal order of mandatory isolation,
with stiff fines & possibly even jail, for being outside their property for any reason, bar a fire etc
The App is now running, but it is a complementary piece of kit, that will only reach those who download and use it fully
There is resistance here, due to memories of Stasi monitoring the population and passed down tales of Nazi control too
Other countries will also have a significant chunk of the population who have no Smart phone / won't download the App / will deliberately leave it at home sometimes