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How COVID-19 Spread from Passengers to Crew Aboard Cruise Ship

A new analysis of a cruise ship quarantined after passengers tested positive for COVID-19 finds food service workers were at the highest risk.

An outbreak of coronavirus disease 2019 (COVID-19) on a Japanese cruise ship in early February made international headlines as a harbinger of the novel coronavirus’ ability to spread quickly.

Now, a new report from the US Centers for Disease Control and Prevention (CDC) is outlining the ways in which the outbreak appears to have jumped from passengers to crew members aboard the Diamond Princess. The outbreak led to most of the ship’s 3700 passengers and crew being quarantined for nearly 4 weeks as public health officials attempted to contain the outbreak. In the end, some 700 people aboard the ship tested positive for COVID-19.

The report shows that the group of crew members who were most hard-hit were the ship’s food service workers, as they were in close contact with patients and with utensils and plates used by the passengers. Out of 1068 crew members on board, a total of 20 crew members tested positive for COVID-19; of those, 15 were food service workers. In total, some 6% of the ship’s 245 food service workers came down with the illness.

Writing in the CDC’s Morbidity and Mortality Weekly Report, corresponding author Hajime Kamiya, MD, MPH, PhD, of Japan’s National Institute of Infectious Diseases, said it was not possible to determine an index patient for the outbreak. However, 1 passenger began developing symptoms on Jan. 22, but remained on the ship until it docked at the Port of Yokohama to start the quarantine on Feb. 3. Another passenger began developing symptoms on Jan. 23 and was disembarked 2 days later.

All passengers with symptoms received tests for COVID-19 upon arrival in Yokohama. Those with positive results were disembarked on Feb. 4 and Feb. 5. Everyone else aboard the ship was ordered to observe a quarantine. For passengers, that meant staying in their cabins. For crew members, that meant performing only essential duties and then staying in their cabins when they were not working.

Among the crew members performing essential tasks were food service workers, who still had to feed the thousands of people on board the ship. The first crew member to show symptoms was a food service worker who developed a fever on Feb. 2. After testing positive, the patient was allowed to leave the ship for treatment of Feb. 4. There is strong evidence that other crew members had also contracted the virus by the time of the quarantine; within 3 days of the start of the quarantine, 7 crew members had begun showing symptoms.

The cruise ship company administered a survey on Feb. 3 asking crew members to self-report fevers. Up until Feb. 6 only crew members with symptoms were tested for COVID-19.

All of the food service workers had cabins on deck 3 of the ship. Of the 5 non-food-service crew members who contracted COVID-19, 1 also lived on deck 3.

Most of the crew member cases could be traced to contact with other sick workers. Three patients said they had contact with sick COVID-19-positive crew members in the days before the onset of symptoms.

Kamiya also reported that 8 crew members who tested positive for COVID-19 had cabin-mates. In the days after the quarantine ended March 4, 5 of those cabin-mates tested positive.

Kamiya concludes that the cruise ship incident is a case study in the importance of a swift epidemiological response, particularly in cases where people are packed closely together.

“Close contacts of persons with confirmed COVID-19 should self-quarantine and monitor their symptoms;” he concludes, “[P]ersons who develop COVID-19 symptoms while on board a ship should be isolated to limit transmission to other passengers and crew.”