The 14-day quarantine on the Diamond Princess was meant to stop the coronavirus outbreak from getting worse.
But as passengers and crew are finally allowed to disembark the cruise ship – described as a “floating prison” by some – questions are being asked about what went wrong on board.
At least 542 cases of COVID-19, the disease caused by the coronavirus, have been confirmed among the 3,711 kept in isolation.
This is the biggest cluster of cases outside China, making the Diamond Princess the only other place where health officials have seen COVID-19 spread easily among people.
Although Japan’s government has repeatedly defended the effectiveness of the quarantine, the ship has seemed to serve as an incubator for the virus.
There are concerns that some of the protocols on board may have been too lax, with three Japanese health workers involved in quarantine checks falling ill themselves.
Dr Nathalie MacDermott, an outbreak expert at King’s College London, said: “Obviously the quarantine hasn’t worked, and this ship has now become a source of infection.”
Scientists believe the disease is mostly spread by droplets when people cough or sneeze, but Dr MacDermott says “there could also be another mode of transmission”.
What the air filtration on board the ship is like, how the cabins are connected, and how waste products were disposed could help explain what happened.
“There’s no reason this [quarantine] should not have worked if it had been done properly,” Dr MacDermott added.
For a fortnight, passengers were told to stay in their cabins and avoid contact with others. Those without a window were allowed to walk on deck for an hour each day provided they wore a face mask and kept their distance.
Dr Paul Hunter, a professor of medicine at the University of East Anglia, suggested that this isolation may not have been as extensive as initially thought.
“It’s difficult to enforce a quarantine in a ship environment and I’m absolutely sure there were some passengers who think they’re not going to let anyone tell them what they can and cannot do,” he explained.
Dr Hunter acknowledged that quarantining 3,700 people in such a confined space is logistically challenging to say the least – adding that infection control procedures may have been more effective if the isolation had taken place on land.
Those on board, including more than 70 British passengers and crew, will be keen to get home and return to their normal lives – but they may not be out of the woods yet.
The Foreign Office has already confirmed that affected Britons will need to complete another 14 days of isolation when they return to the UK, and similar measures have been adopted by other countries too.
Dr Hunter added: “Given how the virus has continued to spread, we have to presume everyone leaving the ship is potentially infected, and therefore they have to go through another two-week quarantine period. Not to do so would be reckless.”
Other scientists have said passengers should have been removed from the ship from the beginning.
Professor Arthur Caplan, who specialises in bioethics at the New York University School of Medicine, said: “Boats are notorious places for being incubators for viruses. It’s only morally justified to keep people on the boat if there are no other options.”
Japanese health officials maintain that the precautions taken on the ship were adequate, and have expressed hope that all on board will get home smoothly and quickly.