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It’s hard to figure out how often people without symptoms spread COVID-19

It’s hard to figure out how often people without symptoms spread COVID-19


Wear a mask anyway

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COVID-19 testing at a pop-up site in New York City.
Photo by Angela Weiss / AFP via Getty Images

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 most people do, though, it’s more important to track them all down.

For months, there have been regular dust-ups over just how likely it is for people without symptoms to spread COVID-19. The latest quarrel stemmed from vague comments by a World Health Organization expert at a press conference. After the expert suggested that asymptomatic transmission was “very rare,” the organization eventually stated that they just don’t have enough information yet. Getting a clear answer will be hard. It takes careful, meticulous detective work to figure out how often people carry the virus without symptoms and how likely they are to pass it onto others when they feel healthy.

It takes careful, meticulous detective work

That’s been done a handful of times in the past few months, says Mark Slifka, a professor at Oregon Health and Science University studying viral immunology. But it’s not enough, and we need far more data to settle the debate.

No symptoms — yet

The confusion started in January when a report in The New England Journal of Medicine said that a German businessman had caught COVID-19 from a colleague visiting from Shanghai. The colleague, the report said, didn’t have any symptoms while she was in Germany. Some experts took that report as the first sign that someone without symptoms (the Shanghai woman) could pass the coronavirus on to someone else (her German colleague).

It turned out, though, that she did have mild symptoms: she felt a bit feverish and tired, with minor aches, during those meetings.

The first challenge researchers face around virus spread from people with no symptoms is reflected in the NEJM mistake: it’s hard to tell if someone truly does not have any symptoms of COVID-19 or if they just have very mild symptoms. Some signs of COVID-19 might be easy to brush off, and someone with a slight tickle in their throat might not think that they’re sick. If scientists are trying to find out if someone with a positive test is symptomatic or not, they often rely on that person to say how they feel. Those self-reported symptoms may not be accurate.

If someone truly does not have symptoms, the next step is checking to see if they ever end up developing them. Many studies of COVID-19 cases check in on people just once, when they initially test positive. If those people don’t feel sick, they can sometimes get classified as “asymptomatic.” Many of them, though, eventually end up developing symptoms later on, which scientists classify as “pre-symptomatic.”

“We should not use the term ‘asymptomatic’ unless you come back at least 14 days later and ask that person, ‘Are you still okay?’”

“We should not use the term ‘asymptomatic’ unless you come back at least 14 days later and ask that person, ‘Are you still okay?’” Slifka says.

In order to get good data on the number of people with the coronavirus who really, truly never feel sick, researchers have to track them for at least 14 days. (The latest people usually get sick after exposure to the virus.) Without that long-term information, the data is no good, said Muge Cevik, an infectious diseases researcher at the University of Saint Andrews, and other researchers in an open letter.

Infectious or just infected

Once scientists have that long-term data, they can start checking to see how often asymptomatic or pre-symptomatic people actually infect others. Studies show that people have high levels of the virus in their throats before they develop symptoms. People who never ended up developing symptoms also had copies of the virus floating around in their noses and throats, one study found, but at lower levels than people who eventually felt sick.

Those studies show that people without symptoms could, theoretically, pass an infection onto someone else. They have copies of the virus in their nose and throat, and those virus copies could make their way over to another person through droplets of spit or the occasional sneeze. That doesn’t mean, though, that they actually will. The amount of virus in a person’s nose is still just an indirect way to measure how contagious they are, Slifka notes. “It doesn’t mean they’re a good spreader.”

Someone who doesn’t feel sick probably isn’t coughing or sneezing, for example, says Abraar Karan, a physician at Harvard Medical School working on the COVID-19 response. “One may argue that once symptoms start, and you’re coughing more, and you’re expelling more respiratory droplets, that that may increase your transmission,” he says.

Information on the amount of virus floating around in people without symptoms and estimates on how many respiratory droplets they produce can be used to estimate how they could spread the virus. Those are just models, though, and they don’t directly answer the question. Finding out, conclusively, if anyone actually caught COVID-19 from someone without symptoms requires detailed tracking and even more testing.

If a group of people is being tested regularly — like in a sports league, for example — it’s easier to re-create the order in which an infection passed from one person to another and when that happened, Karan says. “If this player became positive on this day, and then played in a game, and these other players became positive on days four and five, you can really trace it,” he says.

Re-creating events within a household where everyone was only tested once is harder. If two members of a family test positive on the same day, and one has symptoms and the other does not, it’s impossible to know which direction the virus spread.

researchers also have to exclude all other ways people could have possibly gotten sick

In order to say a person got sick because of someone else who tested positive but didn’t have symptoms, researchers also have to exclude all other ways they could have possibly gotten sick, noted Natalie Dean, an epidemiologist at the University of Florida, on Twitter. That’s hard to do in places with high rates of infection. If the virus is circulating through a community, that person could have picked it up by touching a doorknob in a public place or at the supermarket, for example.

A few research groups have managed to sort through all the clutter. One such study followed people who contracted the virus while working at a call center in South Korea. During the investigation, some workers had symptoms, some developed symptoms later, and some never developed symptoms. No one who came into contact with the workers when they did not have symptoms ended up catching the virus. Another detailed study that carefully traced the path of infection for 157 people with COVID-19 in Singapore found that only 10 were infected by someone who hadn’t yet developed symptoms.

No good answers

That early data suggests that, even if they have the virus in their nose, people who don’t have symptoms yet or who never get symptoms spread the coronavirus less frequently than people with symptoms. The studies are small, though, and aren’t enough to say for sure who is or isn’t likely to spread COVID-19.

They also don’t change what scientists know: some people do spread the coronavirus before they develop symptoms or without developing symptoms at all. Before there’s more evidence, that’s enough for public health experts to recommend that everyone wear masks, even if they feel fine, in an effort to stop the spread of the virus. “We have to play it as conservatively as possible,” Karan says.

“We have to play it as conservatively as possible.”

The coronavirus is still very new, and scientists still have a lot to learn about it.

“We’re still figuring this out, we don’t have complete certainty on the exact amount of spread from pre-symptomatic cases, or asymptomatic, or symptomatic,” Karan says. “That’s why, while we’re still learning about the proportion of transmission by these different groupings, that we wear masks in high risk situations very consistently.”

While we’re taking those steps, researchers should keep searching for better data, Slifka says. In areas with low rates of COVID-19, public health officials can do the detailed contact tracing necessary to figure out if someone caught the virus from someone without symptoms. The more testing states and cities do, the more likely they are to find the people who are walking around carrying the virus without any symptoms.

“We should be monitoring each of the cases, pre-symptomatic and asymptomatic and symptomatic, and checking for that secondary attack rate,” Slifka says. “Then we’ll have an answer.”