Researchers have used mathematical models to assess the effectiveness of the public health measures taken to contain the COVID-19 outbreak on the Diamond Princess cruise ship. The conclusion? More cases could have been prevented if those aboard had been allowed to disembark.

On February 3, 2020, 10 people aboard the Diamond Princess cruise ship tested positive for the new coronavirus, SARS-CoV-2. Each had developed the resulting illness, COVID-19.

By February 4, the people with COVID-19 had been isolated from the rest of the passengers.

The case that prompted this on board outbreak involved a person who had been aboard the ship between January 21 and January 25.

After this passenger had disembarked in Hong Kong and received a COVID-19 diagnosis, Japanese authorities decided to prohibit any other passengers on the ship from disembarking on Japanese land.

When the ship reached Yokohama, Japan, local authorities placed it under quarantine. Those who tested positive for COVID-19 were isolated in a hospital. The remaining crew members and passengers were not able to disembark until February 19.

By the time the authorities lifted the quarantine, 17% of the people aboard the ship had contracted the SARS-CoV-2 virus.

Could the number of people infected have been lower if those aboard had been allowed to disembark earlier? New research suggests so.

In fact, the new paper, which appears in the Journal of Travel Medicine, finds that the number of COVID-19 cases would have been more than eight times lower if the ship had been evacuated in a timely manner.

## Studying alternate scenarios

Joacim Rocklöv, a professor of epidemiology at Umeå University, in Sweden, is the principal investigator of the new research.

The scientist explains, “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.”

By February 20, 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 commonly employed to study infectious diseases to estimate the “basic reproduction number from the initial period of the outbreak.”

The model that they used for this is called the Susceptible‐Exposed‐Infected‐Removed/Recovered (SEIR) model. A basic reproduction number is “the expected number of secondary cases produced by a single (typical) infection in a completely susceptible population.”

The researchers also used models to estimate what would have happened if no countermeasures had been taken, as well as to compare scenarios of evacuating the ship sooner, compared with later.

## Evacuation would have led to 76 cases, not 619

The basic reproduction number was initially four times higher on the ship than it was in the epicenter of Wuhan, China, where the new coronavirus outbreak first started, write the authors.

“A probable cause is how close people stay to one another on board a vessel,” explains Prof. Rocklöv.

After determining the initial basic reproduction number of 14.8, the authors estimated that if no measures had been taken, 2,920 of the 3,700 passengers and crew — or 79% — would have contracted the virus.

“Isolation and quarantine, therefore, prevented 2,307 cases and lowered the [basic reproduction number] to 1.78,” the researchers continue.

However, they note, “An early evacuation of all passengers on February 3 would have been associated with 76 infected persons in their incubation time.”

## A questionable decision

Prof. Rocklöv and colleagues conclude: “The cruise ship conditions clearly amplified an already highly transmissible disease. The public health measures prevented more than 2,000 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 [contracting the] infection.”

“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,” comments the study’s principal investigator.

“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