Coronavirus: Ship quarantine raised cases from 70 to 700, four times higher than worst infection rate on land
The cruise ship, Diamond Princess, which was quarantined for two weeks in Japan, resulted in more coronavirus infected passengers than if they would have disembarked immediately.
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, according to a study conducted at Umea University in Sweden.
A probable reason is how close people stayed to one another onboard a vessel, says principle author Joacim Rocklov, professor of epidemiology at Umea University, in the analysis.
The team found that 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. The analysis shows that around 70 passengers would have been infected, a number that greatly falls short of the over 700 patients 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 Rocklöv.
The outbreak on the ship was traced to a Hong Kong passenger who disembarked on January 25 and found to have carried the virus. After docking near New Taipei City, on January 31, the ship arrived in Yokohoma, Japan. By the following day, the Japanese health ministry ordered a 14-day quarantine for everyone on board and rushed to close its ports to all other cruise ships.
The ship was put in quarantine till February 19. By February 20, the decision was made to evacuate and more than 3,000 passengers left the ship, mostly air-evacuated by their respective countries.
There are now over 700 confirmed COVID-19 cases linked to the Diamond Princess.
A British man became the first UK citizen to die from the new coronavirus after being infected on the Diamond Princess cruise ship in Japan. He is the sixth passenger to die from COVID-19, according to Japan's health ministry.
The researchers in the current study estimated the basic reproduction number from the initial period of the outbreak.
“Cruise ships carry a large number of people in confined spaces with relative homogeneous mixing. On February 3, 2020, an outbreak of COVID-19 on cruise ship Diamond Princess was reported with 10 initial cases, following an index case on board around January 21-25. By February 4, 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,” says the study published in the Journal of Travel Medicine.
The experts modeled that an evacuation by February 3 would have resulted in 76 latent cases (cases during the incubation time).
At the same time, the study also shows that if the precautionary measures of isolating potential carriers had not been carried out on board, more than 2,000 people would have been infected.
“Based on the modeled initial reproduction rate of 14.8, we estimated that without any interventions within the time period of January 21-February 19, at least 2,920 out of the 3,700 (79%) would have been infected. Isolation and quarantine, therefore, prevented 2,307 cases and lowered the reproduction rate to 1.78. We showed that an early evacuation of all passengers on February 3 would have been associated with 76 infected persons in their incubation time,” says the study.
The Japanese government has been questioned over its decision to keep people quarantined on the ship, given the tight quarters and the difficulty of isolating sick people from the healthy. Six government quarantine workers contracted the virus, raising questions about the protective measures used.
Last week, a Japanese infectious disease specialist had criticized the way Japan’s government has handled the COVID-19 crisis aboard the ship.
“Conditions on board the Diamond Princess were “violating all infection control principles” and “completely chaotic,” the scientist, Kentaro Iwata of Kobe University, said in a YouTube video posted on Tuesday evening,” reports Science.
In a press conference on February 24, Japanese officials said the isolation was effective to some degree, but not perfect. That is the analysis of the technical committee members.
Yosuke Kita, a senior coordinator at the Ministry of Health, Labor and Welfare, said the crew members could not be isolated completely, reported CNN.
According to CNN, he said, "Unfortunately to maintain daily life of the more than 3,700 passenger cruise, we needed help, we needed support from cruise members to maintain the daily life.”