One problem is even governments support only the duopoly of smartphone makers. For example, try downloading a "Covid App" for a Linux phone.
For tracing, at least Sailfish OS has an app called Contrac [1]. I looked into porting it over to other PinePhone OS, but with the CRUST and deep sleep, it would not really be useful there anyway (as it would not be active most of the time).
Regarding QR certificates, there’s Vakzination [2].
On iOS PassBook would be the ideal place for this, not a separate app.
I do have mine on my phone but just in case.. Here we don't need the code unless we go to the disco, and I'm too old for that anyway.
Either way I'm vaccinated so code or not I've done the right thing :)