In one cruise-ship coronavirus outbreak, more than 80% of people who tested positive for COVID-19 did not show any symptoms of the disease, according to a new paper published in the journal Thorax.

The research shows just how prevalent asymptomatic transmission of COVID-19 may be—a reality that both suggests official case counts are drastic underestimates, and emphasizes the importance of practicing social distancing even if you feel healthy.

Researchers have known for months that asymptomatic transmission of COVID-19 is possible and common, but without population-wide testing, it’s been difficult to estimate how many people get infected without showing symptoms. The new paper provides an example of how widespread asymptomatic transmission can be, at least in a contained environment.

The (unnamed) cruise ship in the new paper left Argentina in mid-March, with plans to travel around the Antarctic Peninsula and over to South Georgia Island in the south Atlantic ocean. All passengers were checked for coronavirus symptoms before departure, and people who had traveled through COVID-19 hotspots were not allowed on board.

Even still, a passenger developed a fever eight days into the trip, triggering a ship-wide lockdown. Crew members and additional passengers began to develop coronavirus symptoms over the next few days.

The ship was not allowed to re-dock in Argentina after the country closed its borders, so it continued on to Uruguay, where eight people were evacuated to a hospital. Uruguayan officials eventually arranged for everyone remaining on board to get tested before the boat docked.

Out of 217 people on board, 128 tested positive for COVID-19—but only 24 of those people showed symptoms prior to testing. The remaining 104 people—81% of those who tested positive—had not experienced any symptoms, the researchers report.

There were also 10 instances of people testing negative even when their cabin mate tested positive, which the authors say suggests there were some false-negative test results.

It’s difficult to extrapolate the results to larger populations, since the people on board the ship were living in close quarters while entirely isolated from outside life for about a month. Even still, the results are a stark reminder of how likely COVID-19 is to spread within communities, and how difficult it can be to tell when it does.

That’s a particularly important lesson to consider as states reopen and nice weather eats away at many people’s resolve to stay home. The virus can and does spread undetected—and an asymptomatic case can still cause serious illness if it spreads to someone else. Until a vaccine is available, the safest way to keep coronavirus from spreading is to keep your distance from others, whether you’re sick or not.

The Coronavirus Brief. Everything you need to know about the global spread of COVID-19

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Write to Jamie Ducharme at jamie.ducharme@time.com.

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