Coronavirus: Ruby Princess passenger tests positive 10 weeks after disembarking

Authorities are trying to work out how a Ruby Princess passenger tested positive 10 weeks after disembarking.

The Ruby Princess cruise ship departs from Port Kembla in NSW in April. Picture: Getty Images

Health authorities in Queensland are scrambling to work out how a woman who was a passenger on the Ruby Princess cruise ship came to test positive to the coronavirus 10 weeks after disembarking the vessel, in a case that raises questions about the extent of testing of the passengers on board.

Urgent contact tracing is under way in the case, with the state’s Health Department attempting to rule out the possibility that the Cairns woman acquired the infection elsewhere in the community.

That was unlikely, according to Queensland Premier Annastacia Palaszczuk. “We’re monitoring this case very closely to work out if it’s directly related to the Ruby Princess or whether it has been ­acquired in Cairns some other way, but at the moment we’re thinking that it is from the Ruby Princess,” Ms Palaszczuk said.

The Queensland Health ­Department provided no information on whether the woman was in a travelling party on the Ruby Princess with others who had also acquired the infection.

Infectious diseases experts said that was ­likely, however, and it was probably not a case of the virus “lying dormant”.

One scenario was that one or more people from the woman’s travelling party had contracted COVID-19, possibly displaying no symptoms, and then passed the disease on to the woman, either ­directly or via a chain of transmission between several people. That would mean there would be back-to-back incubation periods that stretched for several weeks.

The incubation period for COVID-19 is usually 14 days or less, but scientific literature out of China has reported cases in which the incubation period was as long as 27 days. In addition, people can be sick with COVID-19 for weeks, and are infectious as long as they are displaying symptoms.

When NSW Health officials boarded the Ruby Princess when it docked on March 8, they did not order testing for all passengers on board the ship, only for those who with respiratory symptoms.

Hundreds of passengers from the Ruby Princess contracted the virus and 22 have died. More than 200 crew were also infected.

“This case just highlights the risk of undetected community transmission happening because somebody who either had very mild symptoms or a very long ­asymptomatic period, or a mild ­infectious period, can get missed,” said Kirby Institute head of bio­security Raina MacIntyre.

“In my view, everyone on board a cruise ship where there is COVID should be tested,” she said. “We know from the Diamond Princess, for example, that they tested everyone and about half of the people who were infected had no symptoms.

“I think the lesson to learn from that is if we ever have another cruise ship docked on our shores with people with COVID, everybody on board should be tested.”

As well as the possibility that the latest Queensland case acquired the infection from someone in her travelling party, there was another potential explanation for the late diagnosis, according to Professor MacIntyre.

“The other possibility is that she was infected on the ship and developed an asymptomatic infection, then it became worse and she started developing symptoms.”

University of Queensland vir­ologist Ian Mackay said it was unlikely the virus had laid dormant before the woman displayed symptoms and tested positive.

“I don’t think it would be likely. I’d say they have acquired it from someone else who was related to the boat,” he said.