They probably need to come up with a scheme where you get a blood test and they check for antibodies + one or more additional relevant markers. If you're at or above appropriate levels you get a natural immunity certification which has a certain duration.

Trick is that they need some data to set the thresholds and validity duration.

It would probably not be practical to just grant X duration from the date of infection, far too much variability in individuals, the lowest common denominator might be only a matter of weeks.