https://onlinelibrary.wiley.com/doi/10.1002/jmv.26958                           80 % of Covid 19. cases are mild or Asymptomatic. This is not the only study that suggests/shows that past exposures to Endemic Coronaviruses produces T cell cross reactivity to covid 19. And can explain why some healthy people who lack this previous exposure can have very serious covid 19.

https://www.sciencedirect.com/science/article/pii/S1386653221001141 This is a serious study on this topic

there is theory, published sometimes in november last year, that very similar virus was in south-east asia some 15-20k years ago. That would explain immunity at the beginning of pandemic