We have understood natural immunity since at least the Athenian Plague in 430 BC. Here is Thucydides:
‘Yet it was with those who had recovered from the disease that the sick and the dying found most compassion. These knew what it was from experience and had no fear for themselves; for the same man was never attacked twice—never at least fatally.’ – Thucydides
We have lived with endemic coronaviruses for at least a hundred years, for which we have long-lasting natural immunity. As expected, we also have natural immunity after Covid-19 disease, as there have been exceedingly few reinfections with serious illness or death, despite a widely circulating virus.
For most viruses, natural immunity is better than vaccine-induced immunity, and that is also true for Covid.
https://brownstone.org/articles/hospitals-should-hire-not-fire-nurses-with-natural-immunity/