Its hard to figure out how often people without symptoms spread COVID-19

In January, early reports out of China started to hint that people infected with the coronavirus were contagious before they started to show symptoms. That’s unusual for a respiratory virus like this one, and it worried public health experts all over the world: it’s much harder to control the spread of a disease if someone who has it can pass it along before they know they’re sick.

It’s since become clear that, yes, it’s possible for people who don’t feel sick to infect other people. What scientists still don’t know, though, is how often it happens. And that matters for managing the pandemic: if only a few people spread COVID-19 when they’re symptom-free, missing them probably won’t have a big impact on the course of the pandemic. If m...

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