Transmission potential of the novel coronavirus (COVID-19) onboard the diamond Princess Cruises Ship, 2020

Mizumoto, Kenji; Chowell, Gerardo

Our findings indicate that the passenger-to-passenger transmission type dominated the transmission dynamics aboard the Diamond Princess Ship. The rapid increase in the overall R t from day 12 (January 31, 2020) to day 19 (February 7, 2020) is greatly influenced by the increase in passenger-to-passenger transmission, and this time period covers the time lag between February 1, 2020 when the index COVID-19 case was reported and February 5, 2020 when the Japanese government requested the passengers to stay inside their cabin after the detection of a cluster of COVID-19 positive cases (National Instatitute of Infectious Disease, 2020; Kumamotonichinichishinb, 2020). Upon the implementation of the quarantine orders, the overall R t and R t for passengers aboard the Diamond Princess Ship declined thereafter, while R t among crew maintained a steady level and only started to decline on day 25 of the outbreak (February 13, 2020) . This is consistent with the fact that passengers staying inside their cabins led to a substantial decline in passenger-to-passenger transmission type except for their interaction with cabin mates. Indeed, the proportion of secondary infections inside the cabins increased from 7% (1/15) on February 6, 43% (3/7) on February 10, 2020 to 100% (1/1) on February 14, 2020 (National Instatitute of Infectious Disease, 2020). Thus, a high proportion of the symptomatic cases after the quarantine gradually shifted from largely passenger cases to crew cases. Indeed, despite their potential risk of acquiring the infection, crews had to continue to work to deliver services to isolated passengers. The distribution of the median Re by transmission type suggests that other transmission types likely contributed to this outbreak to some extent (Fig. 3) . Because one quarantine officer, one nurse, and one administrative officer contracted COVID-19 aboard the Diamond Princess Ship, the infection risk continued to be significant inside the Diamond Princess Ship.