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Covid-19 Is Spreading Far and Wide From People Who Don't Feel Sick, New Research Finds

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The Miami International airport on March 15, 2020.
The Miami International airport on March 15, 2020.
Photo: Getty Images

More and more research is starting to confirm a troubling suspicion scientists have had about covid-19, the disease caused by the new coronavirus: People without symptoms can not only spread the infection but are actually fueling its pandemic spread across the globe.

As recently as February, the consensus among public health experts was that the greatest danger of catching covid-19 came from being around people whose symptoms had already begun. The period when a person is infected but not yet feeling sick, called the incubation period, seems to typically be five days to two weeks, and it wasn’t clear whether people could spread the infection during that time. There had been reports of asymptomatic transmission in China, where the outbreak began, but also some confusion. In early February, a report looking into the first few cases locally spread in Germany that attributed them to an asymptomatic index case was criticized after it turned out that the patient who sparked that outbreak wasn’t asymptomatic after all.

Since then, evidence has continued to stack up that people are regularly spreading covid-19 without having symptoms, either as carriers who experience no or very mild symptoms or before they actually become sick, otherwise known as being presymptomatic.


Some researchers have found that people are most contagious before and during the first week that symptoms start, meaning they shed infectious viral particles from their nose and mouth. A cluster of 82 reported cases in Massachusetts at the biotech company Biogen, CNN reported this weekend, is thought to have been sparked by three employees who didn’t show any symptoms at the time they spread the infection. Researchers trying to model the outbreak’s path in other countries have also found substantial evidence of asymptomatic transmission.


One preliminary study on the preprint website MedRxiv released Sunday estimated that around 48 percent of cases transmitted in Singapore and 62 percent of cases in Tianjin, China were spread by presymptomatic people. Though this study hasn’t been peer-reviewed, it echoes the findings of a new study that was published in Science today, which found that 86 percent of all infections in China prior to January 23 (when heavy travel restrictions were enacted) were undocumented.

“These undocumented infections,” the authors wrote, “often experience mild, limited, or no symptoms and hence go unrecognized, and, depending on their contagiousness and numbers, can expose a far greater portion of the population to virus than would otherwise occur.”


The implications of this research are, to be blunt, deeply worrying. People with mild to no symptoms are likely less contagious than those who are in full-blown illness, but because there are so many more of these hidden infections, it’s those people who continue to interact with others and drive the outbreak forward. By the researchers’ estimations, the undocumented infections in China were about half as contagious as people with documented covid-19, but they still accounted for 79 percent of documented infections.

If the team’s estimated ratio of documented-to-undocumented cases is roughly accurate everywhere, then there’s already been a million cases of covid-19 worldwide and counting, based on the nearly 180,000 reported cases as of March 16.


Many of these cases may have already recovered, and countries such as South Korea and China do appear to have beaten back their outbreaks for the time being. Nonetheless, the pandemic as a whole is only getting worse. Most affected countries, including the U.S., have barely started to implement the sort of drastic but essential measures needed to slow down transmission in our communities, while many cities’ bars and restaurants continued to hold large gatherings over this past weekend

The findings reinforce, more than ever, the importance of social distancing. On Sunday, the Centers for Disease Control and Prevention recommended to the public that no one should attend gatherings of 50 or more people for at least the next eight weeks, and today the White House recommends people avoid gatherings of more than 10 people. But the CDC’s information page on covid-19 still tells people that asymptomatic transmission “is not thought to be the main way the virus spreads”—a statement that will surely need to be corrected soon.