Several studies have shown that people infected with Covid-19 tend to have T cells that can target the virus, regardless of whether they have experienced symptoms. So far, so normal. But scientists have also recently discovered that some people can test negative for antibodies against Covid-19 and positive for T cells that can identify the virus. This has led to suspicions that some level of immunity against the disease might be twice as common as was previously thought.

Most bizarrely of all, when researchers tested blood samples taken years before the pandemic started, they found T cells which were specifically tailored to detect proteins on the surface of Covid-19. This suggests that some people already had a pre-existing degree of resistance against the virus before it ever infected a human. And it appears to be surprisingly prevalent: 40-60% of unexposed individuals had these cells…

The central role of T cells could also help to explain some of the quirks that have so far eluded understanding – from the dramatic escalation in risk that people face from the virus as they get older, to the mysterious discovery that it can destroy the spleen.

Deciphering the importance of T cells isn’t just a matter of academic curiosity. If scientists know which aspects of the immune system are the most important, they can direct their efforts to make vaccines and treatments that work.