Well this is to keep people out, not keep people in to do what Germany did on the left.
I don’t see it violating anyone’s Constitutional rights because there is no Constitutional right for me or you to visit any country. We get our US Passport and we rely on other countries to respect the US passport to let us in or not. If A country doesn’t want US citizens to visit, that is their right. If countries allow US citizens to visit but require a COVID-19 vaccine to do so, well then that is your choice to get it or not.
I plan to visit Italy again as soon as I can so if Italy requires it, I will get the vaccine.
But this could evolve into needing this health or vaccine passport to travel domestically.