Immune to Covid? It’s Possible — But 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. But the challenge trial and studies of household contacts show some people are hard to infect. In one study, only 38% of unvaccinated people living with an infected person caught the virus. That number was 25% for the vaccinated.