The Chinese study suggests that some Europeans and Americans might have been infected with a milder strain that didn’t cause severe symptoms. That may help explain why flare-ups didn’t occur outside Wuhan earlier in the winter. And the two studies taken together raise the intriguing possibility that more people were exposed and infected than epidemiologists commonly assume. This seems especially probable since scientists are now discovering that many asymptomatic or mild cases don’t develop Covid-19-specific antibodies.

One reason is that some people have underlying T-cell immunity from past coronaviruses that can help them vanquish the novel virus without developing antibodies. Several studies have found that even people who were never infected with Covid-19 nonetheless have “memory” T-cells—the immune system’s fighters—from past coronavirus infections, which attack the new virus. A La Jolla Institute for Immunology team has detected residual T-cells in about half of blood samples collected between 2015 and 2018.

Recent studies have also found that many people with mild or no symptoms who test positive for Covid-19 later don’t show antibodies when tested. Patients with mild symptoms produce a weaker antibody response than those who get more severely ill. Most antibody tests are primed to minimize false positives, but as a result are less sensitive.