As someone who would enforce vaccination certificates, I would kick such people out of the healthcare system or make them pay more for living obese unhealthy lives.
Do you see how using such a narrow lens could be unwise? How many boosters are you willing to tolerate to maintain your certificate, 2, 5, 10? How about we make them a monthly requirement, any issues?
The certificate does not stop you catching or spreading covid, but it will make you feel more 'safe' and better about yourself.
It is much harder to take care of yourself when you have less money, also. Typical example from the US is eating cheap, subsidized burgers instead of healthier veggies.
Maybe tackling the stress from the "modern" capitalist lifestyle might help everyone to lead a healthier life. :-)