Asymptomatic COVID-19 cases on the Diamond Princess cruise ship

Asymptomatic COVID-19 cases on the Diamond Princess cruise ship. Spencer EA, Heneghan C.

Published on June 28, 2020

Reference Mizumoto K, Kagaya K, Zarebski A et al. Estimating the asymptomatic proportion of coronavirus disease 2019 (COVID-19) cases on board the Diamond Princess cruise ship, Yokohama, Japan, 2020 Euro Surveill. 2020;25(10):2000180. doi:10.2807/1560-7917.ES.2020.25.10.2000180
Study type
Country Japan
Setting Cruise Ship
Funding Details Japan Society for the Promotion of Science (JSPS); Oxford Martin School Programme on Pandemic Genomics, and NSF-NIH-USDA Ecology and Evolution of Infectious Diseases program.
Transmission mode Close contact, person to person
Exposures Cruise Ship

Bottom Line

18% of cases were estimated to be asymptomatic on board the Diamond Princess cruise ship

Evidence Summary

As of the 20th February, 634 persons (among 3,063 tests) onboard tested positive for COVID-19.  Within these 634 cases, 306 were symptomatic and 328 were asymptomatic.

The estimated delay-adjusted asymptomatic proportion was 17.9% (95% credible interval: 15.5 to 20.2%).

What did they do?

On 5 February 2020, in Yokohama, Japan, a cruise ship hosting 3,711 people underwent a 2-week quarantine after a former passenger was found with COVID-19 post-disembarking. 

Laboratory testing by PCR was conducted, prioritising symptomatic or high-risk groups.  The authors conducted statistical modelling to derive the delay-adjusted asymptomatic proportion of infections, along with the infections’ timeline.

Study reliability

Especially in the early phase of the quarantine, PCR tests were focussed on symptomatic cases. If asymptomatic cases were missed as a result of this, it would mean an underestimation of the asymptomatic proportion.

The sample is not highly representative of the general population, being typically over age 60. The presence of symptoms in cases with COVID-19 may correlate with other health factors such as cardiovascular disease, diabetes, and/or immunosuppression, which are not measured here.

Clearly defined setting Demographic characteristics described Follow-up length was sufficient Transmission outcomes assessed Main biases are taken into consideration
Yes Yes No Yes Yes

About the authors

Carl Heneghan

Carl is Professor of EBM & Director of CEBM at the University of Oxford. He is also a GP and tweets @carlheneghan. He has an active interest in discovering the truth behind health research findings

Elizabeth Spencer

Dr Elizabeth Spencer; MMedSci, PhD. Epidemiologist, Nuffield Department for Primary Care Health Sciences, University of Oxford.