You can always chase people who sneeze on the streets(hang around a hospital maybe?), get your covid, test positive, quarantine yourself for 2 weeks and hope for the best as an immuniser or a natural booster.

The vaccines on the other hand are much more practical and safer. They seem to lose some efficacy over time against getting covid but the results against getting seriously ill are very promising. Which probably means, if you get Covid-19 naturally later on, it would be a natural booster with much lower risk compared to a non-vaxxed person.

All immunity is natural; vaccines don’t install a new immune system.