More than 300 people on board the Ruby Princess cruise ship could have already contracted coronavirus by the time it docked in Sydney, according to an expert in epidemiology.
As of Tuesday, 133 people (107 in New South Wales and 26 interstate) have been diagnosed with Covid-19 after leaving the ship, which docked in Sydney’s Circular Quay on Thursday 19 March. All 2,700 passengers were allowed to disembark and return home and told to self-isolate.
Associate professor Ying Zhang from the University of Sydney said it was likely that the number of confirmed cases would “double or triple” in the next few days.
That means hundreds of people would already have had the disease on the day it docked in Sydney, due to the virus’s 14-day incubation period.
Four people from the Ruby Princess tested positive for Covid-19 on Friday, and that number rose to 26 on Sunday, 48 on Monday, and then 133 on Tuesday.
Zhang said the total number of cases would be determined by what precautions the passengers and crew took while sailing, before the first cases were diagnosed.
“It depends on what medical and precautionary measures they had already taken on the ship, before they left, because they were all infected on the ship,” she said. “It’s a 14-day incubation period.”
Elisa Mccafferty, a passenger on the ship, told Guardian Australia earlier that people were told to fill in a form declaring if they had a cough or fever, but the Ruby Princess did not inform passengers that there were suspected cases of coronavirus on board.
Zhang said that the environment of the cruise ship spread the disease more widely than it would in the community.
“The ship is a relatively enclosed environment. I don’t know how their routine daily lives would be on the ship. I would assume they have a set time for dinner together, they would have centralised air conditioning, and could only go outside if the weather allowed.
“Basically that is a really good environment for the virus to spread. One confirmed case can infect two and a half people. In the contained area of a cruise ship, it would be much higher, I can’t give an exact figure.”
However, she said that once in the community, passengers with Covid-19 would not transmit the disease if they followed quarantine requirements.
“If they are all isolated and follow the instructions, it won’t transmit,” she said. “If they are not doing that well, we would apply the community transmission average of 2.5. We don’t assume they have more severe symptoms or the virus can be stronger [among Ruby Prince passengers than the average].”
On Tuesday, one of the initial four diagnosed patients, a woman in her 70s, died in a Sydney hospital. She had been taken directly from the ship to hospital on Thursday when it docked.
On Saturday, the NSW health minister, Brad Hazzard, said in hindsight he would not have allowed the 2,700 passengers on board to disembark.
NSW Health told Guardian Australia that “all passengers on the Ruby Princess were instructed to remain in isolation and seek testing should they develop symptoms”.
“Contrary to some public statements made, every cruise liner that has entered the port of Sydney since February 14 – more than 60 – has been the subject of an assessment well beyond federal requirements,” a spokesman said.
However, Zhang criticised the decision to let all 2,700 passengers disperse into the public.
“I was a bit angry when I saw the government let the people go home on public transport,” she said. “I know they are following the instructions to self-isolate for 14 days, but on the way back home, they can take public transport? I think it is a bit tricky.”
Some 63% of the passengers on the Ruby Princess were Australian residents, 20% were American residents and the remaining 17% were from other countries.