Covid-19 cases on cruise ships possibly significantly underestimated, study finds

Some four-fifths of Covid-19-infected personnel on the cruise ship Greg Mortimer had no symptoms, illustrating the challenges for post-cruise infection control, a study published on Friday May 29th has claimed.

The findings follow another study, covering the January breakout on cruise ship Diamond Princess, which concluded that 75% of those infected on board showed no symptoms.

The study, published in medical journal Thorax, and which was written by three people who were on the cruise, noted that no Covid-19 cases were detected prior to embarkation. The first reported fever occurred eight days into the voyage, well after the vessel had crossed the Drake Passage and arrived off Antarctica.

Professor Alvin J Ing of the Faculty of Medicine at Macquarie University, Christine Cocks of the Oncology Trials Unit at Sunshine Coast University Hospital and expedition physician Dr Jeffery P Green, said that after the first suspected case, stringent isolation protocols were implemented, including cabin confinement, PPE for all contact with ill passengers and room-to-room meal service.

Three crewmembers presented with fever on day 10, two passengers and one crewmember on day 11, and three more passengers on day 12. Most of these symptomatic individuals recovered by the time the ship reached Montevideo.

More illnesses were reported over the following days. By day 22, eight passengers had been medevaced in serious condition.

The Uruguayan Ministry of Health provided everyone on board with PCR lab testing (conducted by global biotech company Atgen). According to the authors, 104 out of all 128 Covid-19-positive personnel displayed no clinical symptoms. Of the 24 others who tested positive with clinical symptoms, one died and four had to be intubated.

About 60% of the people on board tested positive (128 out of 217). All passengers were kept in quarantine until they tested negative, at which point they were permitted to board repatriation flights.

As the authors observed, the case of the Greg Mortimer is useful because her personnel were fully quarantined, with all of them being tested using a PCR test. The authors concluded that “the prevalence of COVID-19 on affected cruise ships is likely to be significantly underestimated, and strategies are needed to assess and monitor all passengers to prevent community transmission after disembarkation.”

The authors also pointed to further evidence of a significant false-negative rate for PCR testing. The post-outbreak test programme on Greg Mortimer returned “discordant Covid-19 results in numerous cabins”. In other words, PCR is likely to undercount the number of people infected.