I'd wager that depends on how the law is written. The court may be unwilling to make scientific conclusions, unless the law is somehow vague on the matter. IOW, if the law states, "every one shall receive the following vaccination," I doubt a court would allow the presence of antibodies to preclude enforcement of such a law.
How would they deal with someone who would have a deathly allergic reaction to the vaccine?
The cynic in me says they would find that risking the death from the vaccine outweighs the risk of spreading the virus, even where that risk is 100%.