I’ve written a bit about variolation, a sort of poor-man’s version of a vaccine. The idea is you intentionally (or accidentally, I suppose) get a small dose of an infectious disease that hopefully won’t make you too sick but will make you sick enough to develop immunity.
Here’s an intriguing idea: Maybe wearing a mask can act as a sort of variolation device, to reduce one’s exposure to the virus and hence reduce the severity of symptoms. This could help explain declining COVID-19-related deaths. See an article by Web MD (which quotes Amesh Adalja) a study by NEJM.