My suggestions are generally useless, but you may want to try to find out whether anyone has a legal right to ask your personal medical history. He may give you a date by which to be vaccinated else youíre out. But when that day comes, you may be able to legally stay silent and there is nothing he can do against the possible implication of unfair dismissal: He will be able to wash his hands of it, and you will be able to wash your hands of it - stalemate. But as mentioned above, my suggestions are generally useless.
Other alternative: Say you are immunosuppressed or immunocompromised and didnít want to tell him earlier, it is kind of private. What can he do? I know there are two of you so it may be kind of hard for you both to say the same thing :-) But if you also say you have Israeli citizenship, he may kinda know you can say anything you want and get away with it.
Take care, Shyaku