Lifestyle Health More Than 80 Percent of Cruise Ship Passengers Who Had Coronavirus Didn't Show Symptoms Researchers say their findings indicate that far more people have been infected with COVID-19 than the numbers show By Julie Mazziotta Julie Mazziotta Twitter Associate Editor, PEOPLE Health People Editorial Guidelines Published on May 28, 2020 04:09 PM Share Tweet Pin Email Photo: PABLO PORCIUNCULA/AFP via Getty On a cruise ship with a high number of COVID-19 cases, more than 80 percent of the patients did not show any symptoms, a worrying sign that the virus can quietly spread without warning signs. Researchers in Australia studied a ship of passengers and crew on a 21-day cruise from Argentina to the Antarctic Peninsula, which left in mid-March, after the World Health Organization declared the new coronavirus a global pandemic. Three days into the trip, they decided to end the cruise early, at their next port docking on day 14. But by the eighth day at sea, a passenger reported having a fever. All passengers were then required to isolate in their rooms until the end of the trip, but the virus spread quickly — of the 217 passengers and crew on board, 128 tested positive for COVID-19. Up to 50 Percent of COVID-19 Cases Never Show Symptoms, Experts Say And of those 128, just 24 showed symptoms. The remaining 104 COVID-19 patients, or 81 percent of the ship, were asymptomatic. The researchers say these results, which have been peer-reviewed and published in the journal Thorax, indicate that COVID-19 is a "silent infection" that has spread at a higher rate than what is known. “It is difficult to find a reliable estimate of the number of COVID positive patients who have no symptoms,” Alan Smyth, professor of child health at the University of Nottingham and joint editor-in-chief of Thorax, said in a blog post about the study. “… As countries progress out of lockdown, a high proportion of infected, but asymptomatic, individuals may mean that a much higher percentage of the population than expected may have been infected with COVID.” WHO Warns That Antibody Testing Does Not Ensure Immunity: ‘There Is Not Enough Evidence’ Smyth added, though, that this may mean that more people have unknowingly had COVID-19 and “these individuals may have immunity.” At this point, however, it is not yet known if people who had COVID-19 are immune to getting it again. The “urgent” need now, Smyth said, is for “accurate” antibody testing worldwide to get a better understanding of how many people have had COVID-19. Though antibody testing is becoming widely available in the U.S., many of the tests may not be accurate, the Centers for Disease Control warned this week. As information about the coronavirus pandemic rapidly changes, PEOPLE is committed to providing the most recent data in our coverage. Some of the information in this story may have changed after publication. For the latest on COVID-19, readers are encouraged to use online resources from CDC, WHO, and local public health departments. PEOPLE has partnered with GoFundMe to raise money for the COVID-19 Relief Fund, a GoFundMe.org fundraiser to support everything from frontline responders to families in need, as well as organizations helping communities. For more information or to donate, click here.