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To know whether social distancing efforts are helping slow the spread of the coronavirus, and to gauge how long we should go on this way, scientists need to know how many people have mild or asymptomatic cases of COVID-19. There could be many such people. Since their symptoms are vague and possibly nonexistent, the case numbers that climb by the day aren’t what they seem. The true numbers may be much higher — or only a little higher.

Understanding the mild cases can help researches get a handle on the spread of the disease — how it’s spreading and how widespread it’s already become.

Jeffrey Shaman, a professor at Columbia University who studies how the environment affects infectious diseases, has gathered evidence that in China, the epidemic was driven by a lot of not-very-sick people. He estimates that about 86 percent of infections early in that outbreak were transmitted by people who never got sick enough to go to the doctor.

The paper he co-authored, published in Science, was popularized as a warning against “silent spreaders” — people who have no idea they are infected. Another study showed that in China, people within one family had symptoms that ranged from undetectable to severe. Random testing done in Iceland showed that 50 percent of people who tested positive had no symptoms.

But more important than determining who has gotten the disease is determining who is giving it to others. Are people without symptoms transmitting the virus? Shaman’s study suggested that about 86 percent of documented infections were picked up from people with undocumented ones. That’s why this issue is worth a closer look, and why I called him.

“You’re getting stuck on this symptomatic/asymptomatic divide,” he told me. Reality is not that clear cut. Symptoms are self-reported and subjective. The symptoms of mild coronavirus are pretty vague except for the fever, which people might not measure if they feel normal. New evidence suggests that those with very mild cases might experience a loss of the sense of smell. The prevalence of this symptom is not yet known — people who can still smell should not assume they are uninfected, but those who suddenly lose that sense should consider themselves sick.

The range of severity is making this pandemic hard to fight — if it were severe in most people, as SARS was, it would be easier to stop the spread. If it were mild in most people, we’d treat it like the flu. But instead it’s extremely severe and deadly in some people and extremely mild — and possibly even silent — in others.

It’s been established that after people become infected, they can walk around for an average of five days and up to around two weeks before they start to feel sick, and it’s not yet clear how many of those days their bodies shed enough of the virus to infect others.

For their new research on COVID-19, Shaman and his colleagues tallied the reported symptoms of people who got the disease in China during the period between Jan. 10 and Jan. 23, right before the government started imposing travel restrictions. The conclusion was that people without documented symptoms were about half as contagious as those with them, and yet they constituted the majority of people who got and spread the disease during that period. Whether they had no symptoms, were pre-symptomatic or had mild symptoms isn’t important. What is, he said, is the fact that many people with the virus felt pretty good, and were moving about society and creating new chains of transmission.

That’s why telling people to stay home if they feel sick isn’t enough — not just because everyone defines “sick” differently, but because people are contagious before they feel sick, and some may not feel sick at all.

Countries that have successfully avoided being overwhelmed by COVID-19 have used extensive testing and contact tracing, so that healthy-feeling people who had been in contact with someone who tested positive would stay home for two weeks just in case. The United States has so far failed on that front — but that’s the next vital step in curtailing this pandemic. In the meantime, everyone has to focus on keeping a safe distance from others to avoid getting or giving the virus.

Science writer Faye Flam is a Bloomberg Opinion columnist.

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