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Diamond Princess is a cruise ship registered in Britain, and owned and operated by Princess Cruises. During a cruise that began on 20 January 2020, positive cases of COVID-19 linked to the COVID-19 pandemic were confirmed on the ship in February 2020. Over 700 people out of 3,711 became infected (567 out of 2,666 passengers and 145 out of 1,045 crew), and 14 people, all of them passengers, died.

On 1 February, the ship called at Naha Port in Okinawa and was quarantined.On the same day, the 80-year-old man who disembarked in Hong Kong six days earlier tested positive for COVID-19. Having a fever, he went to the hospital and got tested. Hong Kong’s Department of Health immediately contacted the agent of the ship’s operating company, but the ship did not immediately inform the passengers and they were notified about that on 3 February, two days later.Over the next few days, the cruise ship had shown and dance parties as usual and also continued to open public facilities that attract large crowds, including fitness clubs, theatres, casinos, bars and buffet-style restaurants.

The authorities immediately decided to isolate all passengers on board for 14 days.On 5 February, the authorities announced positive test results for SARS-CoV-2 for ten people on board, the cancellation of the cruise, and that the ship was entering quarantine for 14 days based on World Health Organization guidelines. A total of 3,700 passengers and crew were quarantined by the Japanese Ministry of Health, Labour and Welfare for what was expected to be a 14-day period, off Yokohama.On 7 February, the total number of people on board with confirmed SARS-CoV-2 infections grew to 61.On 9 February, 6 cases were detectedbringing the total to 135.

On the morning of 17 February (Japan time), two U.S. government-chartered planes departed for the United States, carrying hundreds of U.S. citizens who were passengers of the cruise ship. Among about 400 cruise passengers, 328 boarded the planes, excluding those who expressed their intention to stay on the ship and 44 under treatment in Japan.The U.S. government initially asked Japan to keep them on board for 14 days based on the CDC guideline despite the proposal by the Japanese government to bring American passengers back home early.The U.S. government, however, changed its policy to return home on 15 February.The first plane landed at Travis Air Force Base in California at midnight on 16 February (U.S. Time), and the other plane landed at Joint Base San Antonio in Texas on the early morning of 17th (U.S. Time).

In early March 2020, Indonesia evacuated 69 Indonesian crew of Diamond Princess, after their COVID-19 test results in Japan were negative. However, an Indonesian naval hospital ship bringing them to Sebaru Island for a mandatory quarantine period had two crew ill. All were tested again; 67 passed the second test, but two did not and were retested with a more accurate, different test, with one negative and one positive result. The 68 people with negative tests disembarked at Sebaru Island for observation and the one positive case was evacuated by helicopter to Persahabatan Hospital.Eventually, all 69 crew members received negative COVID-19 test results.

A preliminary report based on the first 184 cases by Japan’s National Institute of Infectious Diseases (NIID) estimated that most of the transmission on the ship had occurred before the quarantine.The cruise line, Princess Cruises, had first assumed there was only minimal risk and had initiated only the lowest-level protocols for outbreaks before the quarantine. By 27 February, at least 150 of the crew members had tested positive for the virus.Dr. Norio Ohmagari, top government adviser and director of Japan’s Disease Control and Prevention Center admitted that the quarantine process might not have been perfect.A crew member reported that many of the crew had been expected to still work and interact with passengers even under the quarantine.

Calculations indicate that an early evacuation could have reduced the case number to just 76 cases, and that the applied quarantine reduced the case number by about 2300 cases.