Immune to Covid? It's possible - but it's a medical mystery

ONE of the more astonishing things that scientists learned from deliberately exposing volunteers to Covid-19 was that nearly half of them never got infected - even when the virus was introduced directly into their noses.

More surprising still, these were people who hadn't been previously infected or vaccinated. This so-called "challenge trial" was originally conceived as a way to speed up vaccine testing, but ended up as a window into the dynamics of the virus.

Two years into the pandemic, about half of Americans still haven't had a documented infection, according to some estimates. Without more in-depth study, it's hard to know how many tested positive on antigen tests at home, had had asymptomatic infections, or perhaps had mild ones that they brushed off as a cold without getting tested.