I am similarly annoyed by the "natural immunity" argument. Whether or not natural immunity is as effective as vaccination (it seems to be?), what I care about and oppose is having to prove medical status as part of daily life. As soon as you shift the argument away from "should we have to show a certificate" to "what gets you a certificate" or faking one, you've already conceded that it's ok to have to show one in the first place. It's not OK and I wish we spent more time on that.