Researchers used mathematical models to determine the efficacy of the measures taken to control the COVID-19 outbreak on cruise ship Diamond Princess. The finale? If those aboard had been allowed to disembark, further cases may have been avoided.
On 3 February 2020, 10 people were screened favorably for the new coronavirus, SARS-CoV-2 on board the Diamond Princess cruise ship. Each had the resulting disease, COVID-19, established.
By 4 February, the COVID-19 people were isolated from the rest of the passengers.
A person who was on board the ship between January 21 and January 25 was involved in the case which prompted this on board outbreak.
After this passenger had disembarked in Hong Kong and was diagnosed with COVID-19, the Japanese authorities decided to ban any other passengers on the ship from landing on Japanese territory.
When the vessel reached Yokohama, Japan, it was placed under quarantine by local authorities. Those who tested COVID-19 positive have been isolated at a hospital. Until 19 February the remaining crew members and passengers were unable to disembark.
By the time the quarantine was lifted, 17 per cent of the people on board the ship had contracted the SARS-CoV-2 virus.
Could the number of infected people have been lower if they were able to disembark earlier onboard? New research suggests that it does.
In addition, the new paper which appears in the Journal of Travel Medicine finds that if the ship had been evacuated in a timely manner, the number of COVID-19 cases would have been more than eight times less.
Studying alternate scenarios
Joacim Rocklöv, an epidemiology professor at the University of Umeå in Sweden, is the principal investigator of the new research.
The scientist says, “If the ship had been evacuated immediately upon arrival in Yokohama and the passengers who tested positive for the coronavirus and possible others in the danger zone had been taken care of, the situation would have looked quite different.”
By 20 February, 619 of the 3,700 total number of passengers and crew members tested positive for SARS-CoV-2.
Prof. Rocklöv and colleagues used a mathematical model widely used in the study of infectious diseases to estimate the “simple replication number from the initial outbreak time.”
The model they used for this is called the Susceptible‐Exposed‐Infected‐Removed / Recovered (SEIR) model. A simple reproduction number is “the estimated number of secondary cases created by a single (typical) infection in a completely susceptible population.”
Researchers also used models to predict what would have occurred if no countermeasures were taken, as well as to compare scenarios of sooner evacuation of the ship compared with later.
Evacuation would have led to 76 cases, not 619
The simple number of replication on the ship was initially four times higher than at the epicenter in Wuhan, China, where the latest coronavirus epidemic first started, the authors write.
“A probable cause is how close people stay on board a vessel to one another,” Prof. Rocklöv explains.
After assessing the initial basic reproduction number of 14.8, the authors determined that 2.920 of the 3,700 passengers and crew— or 79 percent — would have contracted the virus if no precautions had been taken.
“So, isolation and quarantine avoided 2,307 cases and reduced the[ basic amount of reproduction] to 1,78,” the researchers added.
However, they note, “In their incubation period, an early evacuation of all passengers on February 3 would have been associated with 76 people infected.”
A questionable decision
Professor Rocklöv and colleagues conclude: “The circumstances of the cruise ship clearly exacerbated an already highly transmitted disease. More than 2,000 additional cases were avoided by public health initiatives, compared with no interventions.”
“However, evacuating all passengers and crew early on in the outbreak would have prevented many more passengers and crew from [contracting the] infection.”
“Our calculations show that they would have contaminated just about 70 passengers. A number that falls significantly short of the more than 600 passengers that resulted in the quarantine,” the principal investigator of the study remarks.
“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.”– Prof. Joacim Rocklöv
For information on how to prevent the spread of coronavirus, this Centers for Disease Control and Prevention (CDC) page provides advice.