An investigation of SARS-CoV-2 infections among Australian passengers on the Diamond Princess cruise ship

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel RNA coronavirus that is highly infectious and causes respiratory disease. It was first reported in late 2019 in Wuhan, China, and was soon transmitted across the world, infecting millions of people. In March 2020, the World Health Organization declared the coronavirus disease to be a pandemic, which is known as COVID-19, and has claimed more than 4.6 million lives worldwide to date. COVID-19 has common pneumonia-like symptoms such as fever, cough, fatigue, a sore throat, and respiratory problems. Some people are even asymptomatically infected, even though they can transmit the infection to others.

Diamond Princess Cruise Ship and COVID-19 Infection

The Diamond Princess Cruise ship left Yokohama (Japan) on 20 January 2020, for a fourteen-day tour with shore excursions in Japan, Hong Kong, Vietnam, and Taiwan. Unfortunately, one of the passengers who complained of mild dry cough was later confirmed to be SARS-CoV-2 positive. In due course, 19% of the passengers were infected with the virus, and thirteen people died.

Japanese officials implemented fourteen days ship-based quarantine, which required all passengers to remain in their cabins, apart from a brief supervised outdoor exercise for their wellbeing. Among the total of 3,711 passengers and crew on board, 6% were Australian citizens. After fourteen days of quarantine, the Australian Government repatriated 156 asymptomatic Australians, four crew members, and six foreign partners. These people were again subjected to another fourteen days of quarantine at Howard Springs, Darwin. Further, in-country medical health liaison assistance was provided to the 46 SARS-CoV-2 positive Australians who had to remain in Japanese hospitals along with their SARS-CoV-2 negative relatives (21 candidates).

The outbreak in the Diamond Princess helped scientists understand the transmission dynamics, changes in the reproductive factor of SARS-CoV-2 in various stages of quarantine, main disease symptoms, and the role of asymptomatic infection.

Retrospective Cohort Study

Although many descriptive analyses and case series of the Diamond Princess outbreak have been performed, there is a lack of research regarding the epidemiological studies and assessment of clinical characteristics among the SARS-CoV-2 positive Australian passengers of the Diamond Princess cruise. This research gap was addressed recently and is now published in the journal Plos One.

In this study, researchers performed a retrospective cohort study including all the Australian passengers who boarded the Diamond Princess on 20 January 2020. This study provided epidemic curves, symptoms, demographics, clinical and radiological signs, risk factors and predicted the time required to clear SARS-CoV-2 infection.

Main Findings

A quarter of the Australian passengers of the Diamond Princess Cruise tested positive for SARS-CoV-2, among which 29% of passengers persistently remained asymptomatic. Among the symptomatic cases, 17% of the passengers showed signs and symptoms before the ship implemented quarantine. Within this quarantine period, another two-thirds of the passengers, who previously did not show any COVID-19 symptoms, developed signs of infection.

Researchers found that the passengers who were closely in contact with others (e.g., cabin mates, before the commencement of the ship-based quarantine, later tested SARS-CoV-2 positive. This indicates a higher possibility of SARS-CoV-2 acquisition for individuals who were in close contact with SARS-CoV-2 positive people, compared to those passengers who remain isolated or were not much exposed to other individuals.

The epidemic curve revealed that SARS-CoV-2 infections were occurring amongst Australians before ship-based quarantine commenced. In the studied cohort, COVID-19 disease peaked around 3–5 days after the quarantine was initiated. This result is in line with a previous report that stated that implementation of the movement restrictions on 5 February lowered the rate of infection among the passengers who were not exposed to an infected individual. Thereby, passenger-to-passenger transmission was reduced with the implementation of ship-based quarantine.

In regards to the clearance of the infection, researchers calculated the median time to the first of two consecutive negative PCR-based SARS-CoV-2 assays to be 13 days for asymptomatic cases and 19 days for symptomatic cases. Results of viral culture indicated that the SARS-CoV-2 virus did not shed beyond 8 days from the onset of the symptom.


The authors of this study reported that the ship-based quarantine had effectively lowered the transmission of the SARS-CoV-2 virus among Australian passengers. However, the possibility of infection was exceptionally higher among individuals who shared cabins or were exposed to a SARS-CoV-2 positive individual.

Researchers suggested that managing an outbreak in a ship is challenging. There is a huge need for proper guidelines for the crew to handle and deliver food to passengers. Additionally, there is a need for access to onshore quarantine and isolation facilities and effective health policies. In the future, non-pharmaceutical interventions (e.g., use of facemasks) and vaccination of travelers would aid the resumption of cruises.

Journal references:
Dr. Priyom Bose

Written by

Dr. Priyom Bose

Priyom holds a Ph.D. in Plant Biology and Biotechnology from the University of Madras, India. She is an active researcher and an experienced science writer. Priyom has also co-authored several original research articles that have been published in reputed peer-reviewed journals. She is also an avid reader and an amateur photographer.


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