Immunity

David Dunn: Audrey, do you remember me ever getting sick? In the three years we've lived in this house? In the old apartment? Before Joseph was born? Before we ever got married?
Audrey Dunn: I...I can't remember.
David Dunn: Don't you think that's kind of weird--not remembering one cold or fever or sore throat? What do you think it means?
Audrey Dunn: Um...I think it means I'm probably too tired to remember.
--Unbreakable

Seroprevalence studies have indicated that the fraction of people who have been infected by COVID-19 is many times higher than the infection rate approximated by active testing programs. These studies also find that more than half of those with positive antibody tests were asymptomatic.

Why do so many people who contract the virus show no symptoms? Moreover, why do many other people who are in the presence of the virus not contract it at all. Pure luck? Social distancing measures? Strong immunity systems?

This paper suggests that a significant fraction of people (maybe 50%+) may have some immunity to COVID-19 because of previous exposure to other coronaviruses. That would explain a lot of what we see.

As an aside, the paper's 20 co-authors has to be close to a record.