© Photographer: Takashi Aoyama/Getty Images AsiaPac YOKOHAMA, JAPAN - FEBRUARY 21: A passenger is seen on the quarantined Diamond Princess cruise ship at the Daikoku Pier on February 21, 2020 in Yokohama, Japan. Passengers who have tested negative for the coronavirus (COVID-19) will be allowed to disembark the cruise ship since Wednesday after two weeks quarantine period. More than 700 people in Japan have been infected with COVID-19. (Photo by Takashi Aoyama/Getty Images)

(Bloomberg) -- Traces of new coronavirus were found on surfaces in cruise-ship cabins for as many as 17 days after passengers left, researchers said, though it wasn’t possible to determine whether they caused any infections.

Researchers looked at the rooms of infected passengers aboard the Diamond Princess, both those who showed symptoms and those who didn’t, according to a study Monday in the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report.

The ship, operated by Carnival Corp.’s Princess Cruises, had more than 700 coronavirus cases. It was quarantined for a time off of Yokohama, Japan, and was the largest outbreak outside of mainland China at one point.

A previous analysis found that the virus remained viable on plastic and stainless steel for up to three days, although levels fell dramatically over time. It was less stable on copper, where no viable virus was found after 4 hours, and cardboard, which was clean after 24 hours, according to the report in the New England Journal of Medicine.

The latest study looked at uncleaned rooms, but other research has found that cleaning the rooms of Covid-19 patients was highly effective at killing the virus.

Coronavirus has forced the U.S. cruise industry to shut down after a series of outbreaks at sea, and policymakers are looking for clues about the safety of the vessels going forward. The episodes have caused cruise line stocks to crater. The industry has been hurt in the past by outbreaks of norovirus, sometimes called stomach flu.

The CDC report also said that passengers on the Diamond Princess mainly spread the virus before the ship went into quarantine, but infections among the crew peaked afterward.

On a separate Princess Cruises ship, the Grand Princess, crew members probably got the virus on an earlier voyage and then passed it on to passengers, according to the study.

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