Why the number of asymptomatic coronavirus cases matters

Coronavirus testing efforts in Boston and Chelsea led researchers to a startling discovery: A large number of people have been walking around with COVID-19 and don’t even know it. In fact, they may run the entire course of the virus without ever experiencing the slightest of symptoms — even as they travel in their communities and potentially infect relatives and strangers.

In Boston, health officials set out to see how the virus was spreading among the homeless population. What they did not expect to find was that over half the people who tested positive did not feel sick at all. And last week, researchers in Chelsea found a similar trend. Of 200 passersby on the street who agreed to have their blood samples taken, 63 tested positive for antibodies that suggested they had been sick with COVID-19. Of those, 25 said they hadn’t felt sick at all.

These so-called asymptomatic cases have wide-ranging implications for what we know about how the virus is spreading. They reveal the true, overwhelming scope. But the fact that so many people have already been infected — and felt fine — also provides a glimmer of hope.