Apple is introducing in iOS 15.1 digital vaccination records using the SMART Health Card specification.

https://9to5mac.com/2021/09/21/covid...allet-details/

https://smarthealth.cards/faq.html

So it's an attempt to have a specification which can be used by anyone. However, it appears to come from an American source so probably won't be accepted outside the US, especially in Europe.

Or for that matter, it seems unlikely to be widely accepted in the US. But if Google as well as Apple both support it, maybe it would help gain adoption. I guess there needs to be an app that would scan the QR codes and authenticate them.