Epidemiology and quarantine measures during COVID-19 outbreak on the cruise ship Diamond Princess docked at Yokohama, Japan in 2020: a descriptive analysis

Abstract

The outbreak of coronavirus disease 2019 (COVID-19) on the cruise ship Diamond Princess docked at Yokohama, Japan was highlighted due to its number of cases in the early stage of the global epidemic when the picture of the virus itself, as well as epidemiological characteristics, were being established. We conducted an observational epidemiological study of the outbreak, focusing on a total of 403 individuals who developed a fever of ≥ 37.5°C from 20 January to 22 February 2020. Quarantine measures are also discussed with a descriptive method. Of a total of 3,711 individuals (2,031 males) from 57 countries, 2,666 (71.8%) and 1,045 (28.2%) were passengers and crew with mean age of 66.0 (range: 2-98) and 36.6 (range: 19-64), respectively. Among 403 febrile individuals, 165 passengers and 58 crew members were diagnosed as laboratory-confirmed COVID-19 cases. Until 6 February, the number of confirmed cases was three or less per day. However, distribution of thermometers on 7 February revealed 43 confirmed cases, and it then started decreasing. The outbreak was initiated from decks for passengers and expanded to areas for crew. As of 17 March, when more than14 days had passed after disembarkation of all passengers and crew, there was no report of forming a cluster of infections in Japan from them. At the time of the initiation of quarantine, the outbreak had already expanded to most of the decks from those for passengers, and the results might suggest the contribution of the set of quarantine measures in unprecedented challenges of the control operation.

Keywords: COVID-19; Japan; disease outbreaks; quarantine; ships.

Figure 1.

Proportion of confirmed febrile COVID-19 cases in each age group. The proportions of the cases ranged from 4.3% to 7.5% consistently across age groups between 20 and 89 years. No confirmed febrile case aged 19 years or younger was observed, while the largest proportion was in the nineties (16.7%), although the small denominator of these age groups should be considered.

Figure 2.

The number of confirmed febrile COVID-19 cases and the implementation date of quarantine measures. The numbers in the figure describes following quarantine measures implemented: (1) isolation of all passengers in their cabins for 14 days was started, (2) crew members were given instructions of appropriate infection prevention and control practice, (3) all RT-PCR positive patients regardless of symptoms are sent to hospitals or other facilities for isolation and care, (4) thermometers were distributed and self-monitoring was requested for all passengers and crew members, (5) a dedicated Fever Call Center was established on the ship, (6) systematic screening of RT-PCR test for COVID-19 was initiated, and (7) on 19 February, disembarkation was started after confirming negative RT-PCR test, no fever, or other physical conditions.

Figure 3.

Outbreak dynamics of confirmed febrile COVID-19 cases stratified by deck numbers. This figure shows the association between the fever onsets of the confirmed febrile COVID-19 cases and the deck where the cases stayed during travel and quarantine. Passengers cabins were located on deck nos. 4, 6, 8-12, and 14, whereas crew members cabins were located on deck nos. 2-7, 12, and 14. This revealed that COVID-19 had already spread to most of the decks before arrival at Yokohama and initiation of isolation strategy in cabins.