The "buy in" issue is why I consider using Denmark (which is where I live) as an initial test area before internationalization: Danes are generally well educated, and they have a high trust in public authorities compared to many other countries. The use of open source software is also widespread in Denmark.

So an app/server solution developed with 100% transparency on GitHub could easily be pushed to everybody by public authorities - under the critical assumption that we can both make it work technically, and make it completely anonymous.

Just for reference, the same "scale problem" is also the challenge with ordinary vaccines, as I understand it:

They don't really need to give 100% protection to each individual. The critical point is that if a large enough percentage of the population has the vaccine, the R_0 parameter of the given disease will drop below 1, and the exponential spreading turns into an exponential decline.