This argument doesn't hold if he's already had covid. you don't put your seatbelt on after the car crash

No, but you can get into multiple car crashes throughout your life. One would hope they take precautions for the future and start wearing their seatbelt.

...Where both the "much" and the "than" parts are miniscule.

Right, but there's also the social responsibility angle. Given that the vaccine is less risky than not getting vaccinated, then I believe you have a responsibility to get vaccinated.