@flaxmeadow This is still being studied by experts as they don't understand exactly how some people do have existing immunity before they even come into contact with Covid 19 . Never mind fully understanding protection after exposure but if you think you do grand .
pubmed.ncbi.nlm.nih.gov/33582369/
Results: As measured with INF-γ production, 80% of the convalescent COVID-19 patients, all IgG-RBD seropositive, had a strong T-cell response. However, unexpectedly, 44% of unexposed healthy controls, all IgG-RBD seronegative, had a strong virus-specific T-cell response with the COVID-19 IGRA, probably because of prior exposure to common cold-causing coronaviruses or other viral or microbial antigens.
www.bbc.co.uk/news/health-59207466
But around one-in-10 had signs of being exposed, but never had symptoms, never tested positive and never developed Covid-fighting antibodies in their blood.
Part of their immune system was able to get on top of the virus before it managed to take hold - what's known as an "abortive infection".
www.newscientist.com/article/2296980-covid-resistant-people-point-way-to-universal-coronavirus-vaccine/