Doctors are learning more every day about how novel coronavirus is spreading across the globe, and the evidence increasingly shows that people with mild or no symptoms are playing a big role in feeding the global pandemic.

That’s a big problem for countries and states looking to ease back on lockdown restrictions because there is no easy way to tell who might be contagious.

It’s also why social distancing is important for everyone, not just people who feel sick.

A study published last week in the journal Science looked at the earliest outbreak in Wuhan, China. The study estimated that roughly 79 percent of COVID-19 patients in Wuhan got the virus from someone who was not showing obvious symptoms. The Science paper estimated that those “undocumented cases” were less contagious than people with symptoms but accounted for more of the spread because there were so many of them.

“These undocumented infections 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 authors conclude.

Bertha Hidalgo, a Ph.D. epidemiologist at the UAB School of Public Health, said the study in question provides useful information, but there’s still much we don’t know.

“The potential for pre-symptomatic or an asymptomatic phase speaks to the importance of social distancing, but also, the need for comprehensive COVID-19 testing,” Hidalgo said. “It also means that we have a relatively poor understanding of the number of people actually affected by COVID-19.”

Hidalgo said there is other evidence out there to suggest COVID is being spread by people who show no symptoms, including by passengers on the Diamond Princess cruise line, and among a family group in China. She also highlighted a study showing one Italian village’s infection rate plummeting after forced isolations for asymptomatic people.

For the Italian study, the village Vo’Euganeo, 50 km west of Venice, went on lockdown February 23 and all residents were tested repeatedly. The authors of that study estimated 50 to 75 percent of their cases showed mild or no symptoms, and the number of infected people in the village plummeted from 88 to seven after 10 days of lock-down and widespread testing in which people who test positive were strictly isolated whether they showed symptoms or not.

“The percentage of people infected, even if asymptomatic, in the population is very high and represents the majority of cases, particularly, but not only, among young people,” said Sergio Romangnani the author of the Italian study. “Isolation of asymptomatics is essential for controlling the spread of the virus and the seriousness of the epidemic.”

Another study in the New England Journal of Medicine found that one Chinese patient who tested positive for coronavirus with no symptoms had similar viral load as symptomatic cases.

This lines up with anecdotal evidence from high-profile cases like U.S. Senator Rand Paul, who tested positive for coronavirus despite reporting mild or no symptoms. In the six days after Paul was tested, before his results came back, he appeared to be going about his normal life, eating lunch with colleagues in the Senate and even using the Senate gym and swimming pool.

Hidalgo said asymptomatic spread of respiratory viruses is “unusual” and a big reason this pandemic is proving so hard to keep in check.

“The potential for sars-cov-2 (the virus that causes COVID-19) to spread in these asymptomatic/pre-symptomatic phases has made containment very difficult, and has warranted mitigation strategies like social distancing to help the control of spread,” Hidalgo said.

Suzanne Judd, also a Ph.D. epidemiologist at UAB, said COVID-19 patients seem to stay contagious longer than patients who have influenza or other similar respiratory illnesses. For influenza, doctors recommend patients stay isolated for 24 hours after the fever subsides.

Judd said for COVID-19 patients should stay in quarantine for 14 days after their test results come back.

“There’s evidence from China and from Italy that you stay infectious much longer than you would with the flu,” Judd said. “So that advice of 24 hours fever-free is not what we’re giving people with COVID-19.”