Estimating the asymptomatic proportion of coronavirus disease 2019 (COVID-19) cases on board the Diamond Princess cruise ship, Yokohama, Japan, 2020

TLDR

A 2‑week quarantine of the Diamond Princess cruise ship, which carried 3,711 passengers, identified 634 COVID‑19 cases by 20 February 2020. The authors used statistical modelling to estimate the delay‑adjusted proportion of asymptomatic infections and reconstruct the infection timeline. They estimated that 17.9 % (95 % CrI 15.5–20.2 %) of infections were asymptomatic, with most cases occurring before quarantine began.

Abstract

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. As at 20 February, 634 persons on board tested positive for the causative virus. We conducted statistical modelling to derive the delay-adjusted asymptomatic proportion of infections, along with the infections' timeline. The estimated asymptomatic proportion was 17.9% (95% credible interval (CrI): 15.5-20.2%). Most infections occurred before the quarantine start.