Study: COVID-19 Cases on Cruise Ships "Significantly Underestimated"
A study published Friday in the medical journal Thorax reports that 80 percent of COVID-infected personnel on the cruise ship Greg Mortimer had no symptoms, illustrating the challenges for post-cruise infection control.
In a detailed narrative, the authors - passenger Prof. Alvin J. Ing of the Faculty of Medicine at Macquarie University, passenger Christine Cocks of the Oncology Trials Unit at Sunshine Coast University Hospital, and expedition physician Dr. Jeffery P. Green - laid out the course of the infection's spread aboard Greg Mortimer. (All three were on board for the cruise.) No COVID cases were detected prior to embarkation, and the first reported fever occurred eight days into the voyage, well after the vessel had crossed the Drake Passage and arrived off Antarctica.
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, and by day 22, eight passengers had been medevaced in serious condition.
As a precautionary measure, 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-positive personnel displayed no clinical symptoms (80 percent). 24 others tested positive with clinical symptoms, including one who died and four who were intubated.
In total, about 60 percent of the people on board tested positive (128 out of 217). All passengers were kept in quarantine until they tested negative. then were permitted to board repatriation flights.
The case of the Mortimer may be unique in that her personnel were fully quarantined and 100 percent tested using a PCR test. Echoing the concerns of the U.S. Centers for Disease Control, the authors concluded that with the high COVID penetration rate aboard the Mortimer and the low rate of detectable symptoms, "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."
In addition, they pointed to further evidence of a "significant" false-negative rate for PCR testing: the post-outbreak test program on Greg Mortimer returned "discordant COVID-19 results in numerous cabins." PCR is known for its accuracy when it does detect the presence of the virus, and it is the current "gold standard" for testing, but it is also known to miss some positive cases - and thereby undercount the number of people infected. (As a result of this shortcoming, the U.S. Navy believes that pre-deployment lab testing cannot guarantee that COVID-19 stays on the pier.)