Per a new paper published in the journal Thorax, the COVID-19 outbreak on one cruise ship showed that over 80% of the individuals who tested positive didn’t have any symptoms. This means that asymptomatic transmission had been occurring. It has been difficult to know if this rate holds true across the board to the lack of widespread testing here in the United States.

The cruise ship that was studied left Argentina in mid-March. It was supposed to travel around the Antarctic Peninsula and over to South Georgia Island in the south Atlantic. Prior to departure, all passengers were checked for symptoms of COVID-19. In addition, anyone who had traveled through hotspots weren’t allowed to board. Despite the precautions, eight days into the voyage, one passenger developed a fever, which caused a ship-wide lockdown. However, over the next few days, more passengers and crew members started to experience symptoms.

By this time, Argentina had closed its borders, so the ship wasn’t allowed to re-dock. Instead, it went to Uruguay. Eight people had to be evacuated to a hospital and the remaining people on board were tested before the boat was allowed to docked. Out of the 217 people on board, 128 tested positive for COVID-19, but only 24 showed symptoms prior to testing. The other 104 positive-testing people had not experienced any symptoms.

Researchers say that it’s difficult to generalize these results to broader populations because the individuals on the cruise ship were living in close quarters and isolated from outside life for almost a month. The important thing that they want people to remember is that the virus easily spreads undetected and those with asymptomatic cases can still give the illness to others. This is why experts recommend social distancing and wearing masks even if you don’t feel sick.