After the novel coronavirus arrived on the Diamond Princess with a passenger in Japan, it spread quickly among crew members who made food for their fellow workers, a study shows.

The first known infected crew member was a food service worker who developed a fever on Feb. 2, tested positive for the virus and was allowed off the ship two days later. By Feb. 9, there were 20 cases among crew who sought medical attention. Of those 20 people, 15 prepared food for other crew members.

“The crew dining area was identified as the primary area of congregation for the crew; passengers did not have access to this part of the ship,” said the study, which was released Tuesday in the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report.

Even after passengers were quarantined on the Diamond Princess, crew members kept working, delivering meals to passengers, doing their regular jobs and staying in their cabins when they were not working, the report said. Any crew members who showed symptoms had to stay in their cabins.

A crew member told The Washington Post in February that he was scared of the risks he faced by having to interact constantly with other workers. “It was my dream job, but now it has turned into a nightmare,” he said.

Ultimately, more than 700 people on the ship tested positive for the coronavirus. The first passengers who tested positive were people who developed symptoms on Jan. 22 and remained on the ship when it arrived in Yokohama on Feb. 3, along with a person who developed symptoms Jan. 23 and got off the ship two days later. That passenger’s case was identified on Feb. 1.

The report addresses only the initial phase of the investigation into the spread among crew members from Feb. 4 to 12. There was no mechanism in place for systematic testing until Feb. 6, according to the report, so only crew members with symptoms who visited the clinic were tested.

The report said interviews with nine infected crew members indicated that the virus apparently spread among people who lived on the third deck and worked in food service, “probably through contact or droplet spread.” Eight of the 20 initially infected crew members shared a cabin with others; five of those eight cabin mates had developed the illness as of March 4.

“This investigation underscores the need for swift epidemiologic investigation as soon as a COVID-19 case is detected in an area or group where a large number of persons gather in a closed or crowded setting (e.g., a cruise ship, music club, health care setting, sports arena, or gymnasium),” the report said. “These settings have been previously associated with infections spread by contact or droplet, such as influenza.”

It also recommends that close contacts of people with confirmed cases should self-quarantine and monitor their symptoms and that anyone who develops symptoms while on a ship should be isolated to limit transmission.

A recent plan by the cruise industry on measures to address the threat includes proposals to conduct daily temperature checks on crew members, to have designated staff monitor passengers and crew for the virus and to put isolation procedures into place.

Last week, global cruise lines announced plans to suspend operations around the world for a month or more in response to the pandemic.

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