We’ve removed our paywall from this article so you can access vital coronavirus content. Find all our coverage here . To support our science journalism, become a subscriber

This week, the White House issued a warning for anyone who recently fled New York City: Quarantine yourself for a full 14 days or risk spreading COVID-19 to a new community — regardless of whether you’re showing symptoms.

“To everyone who has left New York over the last few days, because of the … number of cases [there], you may have been exposed,” said Deborah Birx, the response coordinator for the White House coronavirus task force, in a news conference on Tuesday.

Officials say new cases of the disease popping up in Long Island suggest fleeing New Yorkers were unknowingly spreading SARS-CoV2 — the coronavirus strain responsible for COVID-19. New York City is now at the epicenter of the outbreak in the U.S., with more than 21,000 confirmed cases as of Thursday morning . Anyone who was in the city recently could easily have been exposed without realizing it.

This is true across the world, and is part of what has made stopping the pandemic’s spread so challenging. Many COVID-19 cases are thought to be mild, and infected individuals with mild or no reported symptoms are still contagious and capable of spreading the virus . Plus, the virus has a long incubation period, with many people not showing symptoms for an average of five days after infection. Together, these two factors result in a lot of people who are infected and spreading the virus without knowing it.

But how does transmission without symptoms actually work? Examining how people can spread the flu and common cold in a similar way may help us understand how people can spread COVID-19 when they don’t feel sick. Recent studies also suggest that understanding asymptomatic transmission of the virus could be key to understanding how COVID-19 is spreading — and, hopefully, to eventually stopping it.

Infected and Feeling Fine

Even when there isn’t a pandemic going on, many people are walking around and going about their regular lives — shedding viruses all the while.

In a 2018 study , Jeffrey Shaman, director of the climate and health program at the Mailman School of Public Health at Columbia University, and colleagues found that about 7 percent of people passing through a New York City tourist attraction in February were shedding some kind of virus as they went. In the study, the researchers asked people passing through a popular attraction about how they were feeling and swabbed their noses. Of those who tested positive for viral infections like human rhinovirus and influenza, about 65 to 97 percent did not report any symptoms.

“I want to note that ‘asymptomatic’ is a very swirly definition,” says Shaman, meaning that symptoms are self-reported and, therefore, subjective. But, in general, it means people who do not report feeling sick but do have a proven viral infection.

‘Shedding’ Viruses

Respiratory viruses, like the novel coronavirus, work by hijacking healthy cells in your nose, throat or lungs (your respiratory tract) and using those cells as factories to produce more viruses. If there is a detectable virus when someone sticks a swab up your nose, that means your oral nasal cavity is excreting the virus, says Shaman, even if you physically feel fine.

These viruses get passed on to others because any time you breathe, cough or even speak, you expel tiny droplets. So, when a virus is reproducing in your respiratory tract, those droplets could contain viral particles. When other people come in contact with those particles, be it in the air or after the droplet lands on a surface, they can become infected as well. “You’re leaking it out,” says Shaman.

But even this theory involves a little bit of guesswork. “The reality is, we don’t really know how respiratory viruses are transmitted,” says Shaman. “We know how they can be, we don’t actually know how they are.” The same is true for the novel coronavirus. We think it spreads through droplets, but there is still too little evidence to say for sure.

Fueling a Pandemic

In a study published in the journal Science earlier this month, Shaman and his colleagues found that undocumented COVID-19 cases were responsible for 86 percent of the spread of the disease in China before the country enacted travel restrictions on January 23, 2020.

The scientists estimated that undocumented cases were about half as contagious as people with confirmed and documented disease. There is likely a correlation between symptom severity and the amount of virus your body is shedding, says Shaman. It stands to reason that if a person is sicker and coughing more, for example, they could be spreading more virus into the community and may be more contagious. But because of the sheer quantity of people with undocumented cases of COVID-19, those people did the “lion’s share of transmission,” says Shaman.

In another recent analysis of COVID-19 spread in China, researchers found that about 10 percent of patients were infected by someone who had the virus but had not yet started to show symptoms. This is not entirely unusual — for some viruses, this pre-symptomatic transmission is possible, says Lauren Ancel Meyers, a professor of integrative biology at the University of Texas at Austin who helped lead the study.

“The fact that there may be some silent transmission for COVID-19 makes it very difficult to contain,” says Meyers. That’s why people worldwide are now taking such extreme social distancing measures to try to get the outbreak under control.

Hopefully, as scientists continue to get a better understanding of this virus, how it spreads and how we could unknowingly be spreading it, we will be able to relax some of the strict social distancing measures currently in place. For now, however, the best we can do is stay home so we don’t become unwitting vectors.