I seriously doubt there will be such a requirement this summer. Hopefully, the travel restrictions will be lifted in combination with a negative test, as with many other countries. I do, however, expect a vaccine to replace testing as they are more readily available.

We are in the same situation. We will travel to our condo in Florida mid June, with 15 nights in Cancun due to the restrictions and then enter the US. I will have received my first dose by that time, Pfizer or Moderna and will get my booster in Florida. It is available for me as I am considered a Florida Resident, since we own property there and stay more than five weeks. This is what all Canadians in our community did when in Florida. I don't know how to get the certificate in Europe but I suspect that it can be entered into the database here as you show them the CDC card.