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A woman wearing a face mask walks her dog in Venice, California, on April 18.
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How people are spreading Covid-19 without symptoms

Silent spreaders are playing a significant role in the pandemic.

We know that not everyone who comes into contact with the novel coronavirus ends up getting sick. But just how many people out there have the virus and are feeling fine — and are perhaps unknowingly spreading it to others?

New data suggests it could be a lot.

For example, 60 percent of personnel aboard the aircraft carrier Theodore Roosevelt who tested positive for the coronavirus appeared healthy at the time, Reuters reported. Other data — from Iceland and elsewhere — have uncovered similar patterns.

With testing capacity still limited, most governments continue to focus on people who are showing symptoms of Covid-19 so that they can be isolated. This is important for slowing the virus’s spread. But it also means that globally, we have lagged in finding people who are carrying the virus but aren’t currently sick — and might be spreading it.

(The early evidence that seemingly healthy people can still infect others with the coronavirus is one of the main reasons behind the US Centers for Disease Control and Prevention’s recommendation that everyone wear a mask in public: so that people who don’t know they have the virus don’t accidentally infect others with it.)

So the big question everyone — including infectious disease experts — has is: How big of a role are these “silent spreaders” playing in this pandemic? It’s certainly significant.

People who feel fine are spreading the coronavirus

Throughout the pandemic, public health officials have focused on the advice that people who have symptoms of Covid-19 should self-isolate. This is crucial, to be sure. But it is also becoming “increasingly clear that there are people who are either asymptomatic or minimally symptomatic who can transmit” the virus, Carlos del Rio, chair of the Department of Global Health at Emory University, told Vox.

In fact, people can start transmitting the virus 24 to 48 hours before they start showing symptoms, he said in a briefing for the Infectious Disease Society of America.

This timing detail is no small matter. And new research on presymptomatic transmission in people who did eventually experience symptoms underscores why.

A study in Nature Medicine of 94 confirmed Covid-19 patients found that people were most infectious right before they started to show symptoms. The researchers obtained data about people who had gotten Covid-19 as well as those who had been around them before and after they got sick. Based on this, they estimated that 44 percent of people who caught the virus from the study’s participants had gotten it from people who felt healthy at the time.

They also found that someone who is mildly sick could have been just as contagious as someone with more severe symptoms. Maybe they aren’t coughing as much as someone feeling more seriously ill, but the virus can still spread through talking, sneezing, and coughing.

And what about people who are carrying the virus but don’t ever get sick — the true asymptomatic carriers? Can they spread it, too?

We simply don’t know yet. Hopefully, further studies will figure this out, because it could lead to more informed decisions about how to eventually end the pandemic.

How many people are carrying the coronavirus and don’t know it?

A new study out of Iceland, published in the New England Journal of Medicine, offers important insight about how prevalent asymptomatic carriers may be. Iceland has tested 6 percent of the entire island nation’s population — the most testing per capita of any country so far.

Our World in Data

Part of the study tested those ill with Covid-19 symptoms, but another part involved testing a substantial sample of the general population feeling well — or at least with no greater symptoms than a mild common cold. Of those who tested positive for the virus, 43 percent were asymptomatic. So it’s possible that, at least in Iceland at that point in time, almost half of the people who had coronavirus didn’t know it.

This percentage likely varies from place to place, depending on the timing and severity of the current outbreak.

For example, two New York City hospitals recently tested more than 99 percent of women who delivered babies in their wards over the course of two weeks. Of the 215 women, about 2 percent had Covid-19 symptoms (all of them tested positive for the virus). And about 14 percent of the symptom-free women also tested positive. This means that about 88 percent of people who had the virus in this group had no symptoms at the time of testing.

So one in eight women admitted to the labor and delivery wards — who seemed healthy — nevertheless was carrying the virus.

Although these studies looked at specific populations at specific points in time, they can provide a general idea of what might be going on elsewhere.

How many people catch the coronavirus and never get sick at all?

We know that people who get Covid-19 start feeling sick anywhere from two to 14 days after they first catch the virus. But there also seems to be a subset of people who test positive for it but never develop any symptoms.

This isn’t that unusual. Other viruses often have many people carrying them who don’t get sick. For example, a study in the UK found that about 77 percent of people who had had the current flu strain never got sick (some studies have pointed to lower rates, which also shows how little we know even about common illnesses).

For the norovirus, a common stomach bug, about a third of people who get it don’t become ill — but can still transmit it to others.

And that number is even higher for other viruses, like polio, which only causes illness in some 5 to 10 percent of infections — but the asymptomatic carriers can still spread it to others, who might get the full-blown disease.

For SARS-CoV-2, the World Health Organization cited the statistic that about 75 percent of people who seem asymptomatic when they test positive for the virus eventually go on to develop symptoms of Covid-19. And a series of recent reports have backed that up.

One study, which examined a nursing home in Washington state early on in the coronavirus outbreak, found that more than half of residents who tested positive (57 percent) had no signs of the illness. One week later, however, more than three-quarters of them had developed symptoms.

The mean time for them to have started feeling sick was just three days after they had gotten tested. This shows just how narrow of a window one-shot testing can be.

Another study, which looked at passengers on the Diamond Princess cruise ship, also tried to account for this timing issue. Initial data showed that of the 634 people on board who tested positive for the coronavirus, slightly more than half were asymptomatic at the time. But researchers were able to follow up with these people (after many did eventually get sick) and, based on that data, estimated that the actual rate of asymptomatic infections was about 18 percent.

And 18 percent is still a large number, especially when expanded to a broader population during an outbreak. That’s roughly one in five people. And based on the WHO numbers, it is one in four.

Researchers still don’t know if — and how much — these truly asymptomatic carriers might be contributing to the spread of the virus. But with such large numbers, answering that question (and taking precautionary actions in the meantime) will be essential to understanding how to most effectively slow down this pandemic.

Identifying silent spreaders is essential

Given that roughly three-quarters of people who don’t show symptoms at the time of testing will probably get ill (and that those people are perhaps most infectious just before they start feeling sick) and given that we still don’t know how much asymptomatic individuals are infecting others, limiting the spread of the virus by seemingly healthy people is crucial.

That is one of the reasons large-scale testing is so powerful. For example, in the Iceland study, public health officials were able to identify about 525 people — in a country of just 364,000 — who were carrying the virus but appeared healthy otherwise (that would be the equivalent of finding about 473,000 asymptomatic or presymptomatic carriers in a small sampling in the US, population-wise).

This information allowed Icelandic officials to take action. After positive test results, people were required to self-isolate until they tested negative — “and all contacts of these participants were [also] required to self-quarantine for 2 weeks,” the authors noted. This sort of broad testing and targeted isolation strategy could help curb the spread of the virus and allow more people to continue with less severe social distancing restrictions.

We’ll need more widespread antibody testing to better understand what’s going on

The trouble with so many of these numbers is that they are snapshots in time. And detailed follow-up, like what happened in the Diamond Princess study, is resource-intensive.

This is one of the reasons testing for the presence of the coronavirus is not — and cannot — give us a clear picture of its true prevalence. Just as it might catch someone before the onset of their symptoms, it might miss someone who has already fought the virus off, whether they knew it or not.

That’s why more widespread antibody testing to see if someone has had the virus in the past will be key in determining the actual proportion of the population that has had the virus. (Most current testing is a different method that looks for the presence of the virus during an active infection.)

With that number, experts will also be able to start modeling how many asymptomatic people are contributing to the overall spread of the virus. This will be “critically important” for ascertaining if they “may be sustaining viral transmission,” del Rio said.

(But this will need to be done thoughtfully. There has been substantial debate over, for example, a recent estimate, based on antibody testing, that the prevalence of the virus in Santa Clara County, California, might be 50-85 times higher than the number of rapid test-confirmed cases.)

Better understanding the true number of asymptomatic carriers and their potential to infect others could impact everything from public health guidelines to how soon we can begin to safely resume more normal activities.

The National Institutes of Health is launching a large-scale study to look at this now. It is enrolling up to 10,000 volunteers from anywhere in the US to be tested for antibodies to the coronavirus.

“These crucial data will help us measure the impact of our public health efforts now and guide our Covid-19 response moving forward,” Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in a statement. You can learn more about enrolling here.

In the meantime, all of these mounting data support continued social distancing practices, mask-wearing, and other preventative measures for everyone — no matter how healthy they feel.

Correction, April 22: An earlier version of this article misstated the percentage of asymptomatic people in an Iceland study who tested positive for Covid-19.


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