In the Swedish study, researchers not only checked 200 participants for the presence of immunological proteins called antibodies produced in response to COVID-19 infections, but also for T-cells which are another virus-fighting component of the immune system. Detecting and evaluating T-cells is a bit trickier than measuring antibodies.

The Karolinska researchers, according to the accompanying press release, “performed immunological analyses of samples from over 200 people, many of whom had mild or no symptoms of COVID-19.” The study tested COVID-19 patients, exposed asymptomatic family members, healthy blood donors who gave blood during 2020, and a 2019 donor control group.

“One interesting observation was that it wasn’t just individuals with verified COVID-19 who showed T-cell immunity but also many of their exposed asymptomatic family members,” said Karolinska researcher Soo Aleman. “Moreover, roughly 30 per cent of the blood donors who’d given blood in May 2020 had COVID-19-specific T cells, a figure that’s much higher than previous antibody tests have shown.”

“Our results indicate that roughly twice as many people have developed T-cell immunity compared with those who we can detect antibodies in,” noted Karolinska Center for Infectious Medicine researcher Marcus Buggert.