Cruise ship quarantine resulted in more COVID-19 infections

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The cruise ship Diamond Princess was quarantined for over two weeks resulting in more coronavirus infected passengers than if they would have disembarked immediately. Rather the opposite to what was intended. This according to a study conducted at Umeå University in Sweden.

“The infection rate onboard the vessel was about four times higher than what can be seen on land in the worst infected areas of China. A probable cause is how close people stay to one another onboard a vessel,” says Joacim Rocklöv, professor of epidemiology at Umeå University and principal author of the article.

After a person travelling with the cruise ship Diamond Princess disembarked in Hong Kong and was tested positive for the coronavirus, Japanese authorities decided to disallow the 3,700 passengers onboard to leave the ship when it reached Yokohama. The ship was hence put in quarantine until 19 February. Passengers who showed signs of illness were, as far as possible, separated from other passengers onboard. When the quarantine in Yokohama in the end was removed and passengers could finally disembark, a total of 619 passengers had been infected by the coronavirus.

“If the ship had been immediately evacuated upon arrival in Yokohama, and the passengers who tested positive for the coronavirus and potential others in the risk zone had been taken care of, the scenario would have looked quite different. Our calculations show that only around 70 passengers would have been infected. A number that greatly falls short of the over 600 passengers the quarantine resulted in. The precautionary measure of putting the entire ship under quarantine was understandable, but due to the high risk of transmission on the ship, the decision is now questionable,” says Joacim Rocklöv.

At the same time, the study also shows that if the precautionary measures of isolating potential carriers had not been carried out onboard, another 2,300 people would have been infected.

Background: Cruise ships carry a large number of people in confined spaces with relative homogeneous mixing. On 3 February, 2020, an outbreak of COVID-19 on cruise ship Diamond Princess was reported with 10 initial cases, following an index case on board around 21-25 January. By 4 February, public health measures such as removal and isolation of ill passengers and quarantine of non-ill passengers were implemented. By 20 February, 619 of 3,700 passengers and crew (17%) were tested positive.
Methods: We estimated the basic reproduction number from the initial period of the outbreak using (SEIR) models. We calibrated the models with transient functions of countermeasures to incidence data. We additionally estimated a counterfactual scenario in absence of countermeasures, and established a model stratified by crew and guests to study the impact of differential contact rates among the groups. We also compared scenarios of an earlier versus later evacuation of the ship.

Results: The basic reproduction rate was initially 4 times higher on-board compared to the R0R0 in the epicentre in Wuhan, but the countermeasures lowered it substantially. Based on the modeled initial R0R0 of 14.8, we estimated that without any interventions within the time period of 21 January to 19 February, 2920 out of the 3700 (79%) would have been infected. Isolation and quarantine therefore prevented 2307 cases, and lowered the R0R0 to 1.78. We showed that an early evacuation of all passengers on 3 February would have been associated with 76 infected persons in their incubation time.
Conclusions: The cruise ship conditions clearly amplified an already highly transmissible disease. The public health measures prevented more than 2000 additional cases compared to no interventions. However, evacuating all passengers and crew early on in the outbreak would have prevented many more passengers and crew from infection.

J Rocklöv, H Sjödin, A Wilder-Smith

Umeå University material

Journal of Travel Medicine abstract

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