Two people in their 80s who were quarantined on the British-owned cruise ship Diamond Princess have died of covid-19 in hospital in Japan. Meanwhile, nearly 3000 people are disembarking from the ship, after being quarantined aboard in Yokohama since 3 February following a passenger testing positive for the disease.
All are free to go after for the new coronavirus, although Japan has asked its nationals to stay home and monitor their health for two weeks. British, US and other passengers flown home by their governments this week will spend another two weeks in quarantine to make sure they are clear. Similarly, the US and South Korea are refusing entry to passengers making their own way home for two weeks.
The people disembarking don鈥檛 include the 634 who during quarantine and were taken to hospital, including, last week, the two people who have now died. Epidemiologists suspect the virus spread during quarantine, which, given the history of viruses on cruise ships, may not be surprising.
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The 2666 passengers and 1045 crew of the Diamond Princess were confined to the ship on 3 February, after an 80-year-old man who disembarked on 25 January tested positive for the virus. From 5 February, passengers were confined to their rooms, except for brief excursions 鈥� in masks 鈥� on deck.
The idea was to keep people apart for 14 days, long enough for even slow-incubating infections to show symptoms. During that time, the Japanese health ministry on people with symptoms or who were at greater risk.
The average incubation time of the virus is five days, so the number of people falling ill because of exposure before quarantine should have peaked by 10 February. There were some early cases, suggesting a surge of exposure just before passengers were confined to their rooms.
But positive tests kept rising, including many for people with no symptoms at the end of quarantine, suggesting new infections. 鈥淐learly there was ongoing transmission,鈥� says David Fisman at the University of Toronto. 鈥淭his is not just people going through an incubation period,鈥� says Justin Lessler of Johns Hopkins University in Maryland.
This isn鈥檛 entirely surprising: cruise ships, , have a troubled history with , especially norovirus, the 鈥�winter vomiting bug鈥� that, like covid-19, spreads in droplets. On 12 February, as positive tests among passengers rose, Japanese officials appeared to recognise the risk by . This offered , and most passengers declined.
Trying to beat the problem of viral outbreaks, the US Centers for Disease Control and Prevention inspects food and sanitation procedures on cruise ships in US waters. Diamond Princess did well on its last CDC inspection in 2013.
The ship then left US waters, but conditions should be like those on similar vessels operated by owner Princess Cruises, says Chris Taylor of Woodborough in the UK, who until recently worked as a doctor on cruise ships. One such vessel, the Caribbean Princess, scored a respectable 95 out of 100 in a January CDC inspection.
Yet this February, the Caribbean Princess had a , probably of norovirus, that reached 12 per cent of passengers. In 2018, that CDC scores bore no relation to outbreaks: high-scoring ships even had slightly more.
So it seems likely that something not included in the inspections spreads viruses on cruise ships. Taylor suspects they are deposited on surfaces everyone touches, like handrails, by ventilation or passengers. If ventilation fans in rooms vent into common exhaust ducts, blowback could spread virus droplets. The cruise line , although an FAQ on its website says .
Fisman notes similarities to a mass viral outbreak in the Amoy Gardens apartment complex in Hong Kong, traced to sewage and airflow. Similar shared plumbing may recently have . Mapping who was infected on Diamond Princess against its plumbing and ventilation systems may be the only way to know for sure.
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