There was no way to trust someone saying "I have natural immunity, I had COVID" when the crisis was happening, even more given how it was used politically to fan the flames for political gain.

Stop thinking about individuals, think about systems, and societies.

Given that people were refusing basic instructions (keeping distance, for example); that people dying of COVID in a hospital didn't believe the disease was real due to political influences; how do you think governments and their healthcare systems would be able to track if someone had natural immunity or not? There's no way, the only answer is: vaccinate as many people as possible to cover for uncertainties.

Again, it's a numbers game, in a fast moving crisis there's no opening for individualised actions, you are part of a larger whole and the larger whole required for people to get vaccinate to stamp off the crisis.