Up to 81 percent of Covid-19 patients may be asymptomatic, cruise ship study suggests

In mid-March, a cruise ship departed from Ushuaia, Argentina, and set sail towards Antarctica following explorer Ernest Shackleton’s 1915 route. The passengers and crew aboard didn't know their ship would become a floating Petri dish of Covid-19.

Eight days after embarking, the first passenger got a fever and the cruise ship instituted isolation and testing measures. Almost two weeks later, 59 percent of the 217 passengers and crew tested positive for Covid-19. Out of that group, more than eight out of 10 had no symptoms.

In a study released Thursday in the journal Thorax, scientists report that these findings suggest 'silent' symptomless Covid-19 infections — the sort experienced by asymptomatic carriers — may be far more common than previously thought.

The cruise offered a unique testing ground for study co-authors Alvin Ing and Christine Cocks, scientists who were on board as passengers, and co-author Jeffrey Green, the expedition physician. They witnessed Covid-19 unfold in the unique, isolated environment in real-time, and began their study before they could disembark.

Their report is another example of the fact that Covid-19 can spread like wildfire on cruises, which often have semi-crowded public spaces, close quarters, and older travelers. The findings can also be extrapolated to other scenarios such as aged care facilities and migrant camps, the study's authors tell Inverse.

"The high rate of asymptomatic Covid-19 patients may mean there may be higher rates of immunity to Covid-19 in the general population than we currently are aware of," Ing, a respiratory and cardiovascular medicine researcher at Macquarie University, tells Inverse. At this point, scientists don't know yet how much protection Covid-19 infection creates against future reinfection.

"Being free of symptoms does not equal being free of infection," Alan Smyth, joint editor-in-chief of the journal Thorax and researcher at the University of Nottingham, tells Inverse. He was not involved directly in the study but published a related commentary.

The researchers documented how Covid-19 spread among cruise passengers and crew and charted the enforced isolation measures put in place.

The ship departed in mid-March, after the World Health Organization had declared Covid-19 a global pandemic, and set off for a 21-day cruise. All passengers had their temperatures taken and were screened for Covid-19 symptoms before setting sail.

Passengers who, in the previous three weeks, had passed through countries where Covid-19 infection rates were already high (China, Macau, Hong Kong, Taiwan, Japan, South Korea, or Iran), were not allowed to board. Hand sanitizing stations were widely available on the ship.

The first week went as planned — they crossed the Drake Passage and explored the Antarctic Peninsula visiting Danco Island, Paradise Bay, Lemair’s Passage, and Deception Island. All passengers were healthy, their health known by regular temperature checks.

On day three, cruise authorities decided to cut the trip a week short due to newly announced international border controls and travel restrictions put in response to the Covid-19 pandemic. They planned to skip the South Georgia leg of the cruise and return back to Ushuaia on day 14.

Everything changed when, eight days into the new route, a passenger exhibited a fever, prompting the whole cruise to commence isolation protocols.

All passengers were confined to cabins and offered surgical masks to wear. Full personal protective equipment (PPE) was used when interacting with feverish patients, and N95 masks were worn for any contact with passengers in their cabins. The crew delivered meals to passengers, but didn't service their rooms.

Over the next four days, nine more passengers and crew members had fevers.

“It was difficult personally to witness passengers and crew become progressively unwell, including my cabin mate," Ing reflects. "I have to thank Jeffrey for his wonderful care and attention on behalf of all passengers and crew, as he worked tirelessly, especially when the second ship physician became unwell.”

Meanwhile, Argentina closed its borders. The cruise then requested disembarkation in the Falkland Islands but was refused. Ultimately, the ship sailed to Montevideo, Uruguay, arriving on day 13. By the time the ship reached Montevideo to dock, most of the initially feverish patients' symptoms had improved.

Subsequently, a few initially feverish patients received rapid Covid-19 tests and tested negative. But the ship wasn't allowed to let passengers off until everyone on board received a nasal swab RT-PCR test.

Over the next few days, a total of eight passengers and crew were medically evacuated from the ship to hospitals in Montevideo, all for impending respiratory failure. They all tested positive for Covid-19.

On day 20, all passengers and crew were tested for Covid-19. Ultimately, 128 out of 217 people tested positive.

Just 24 Covid-19 positive patients actually showed symptoms. Sixteen had fevers and mild symptoms, while four required intubation and ventilation. One person died due to Covid-19 complications.

There were 10 cases where two passengers sharing a cabin had conflicting test results. This suggests early, rapid testing isn't always reliable, and RT-PCR tests may have a significant false-negative rate, the authors explain.

Overall, 81 percent of those who tested positive for Covid-19 were asymptomatic. This doesn't quite jibe with what we know about asymptomatic patients so far: The CDC reports 35 percent of infected people may be asymptomatic. Meanwhile, another cruise ship study, this one focused on the infamous Diamond Princess, suggests 18 percent of infected people were asymptomatic — but not all passengers and crew were tested. This study quadruples that number.

While the “population” on the cruise ship was small, every single member of this population was tested. This makes this cruise a unique “natural experiment” that can't be repeated elsewhere, Smyth says.

"There is no reason why the passengers and crew on the ship are any more likely than the general population to be infected with Covid-19 but have no symptoms," he adds.

"It is therefore very plausible that the rate of 81 percent infected but symptom-free might apply to the general population. This means that many more people may be potentially immune than was first thought."

Currently, testing hasn't fully captured how many asymptomatic Covid-19 positive people are out there. But this study suggests the number may be higher than anticipated, and testing only symptomatic people may be missing a huge proportion of infected people.

"Testing just symptomatic subjects in such an at-risk environment is likely to miss a significant population with Covid-19, and may thus promote community transmission if untested or unmonitored," Ing says.

"If we are serious about preventing a second wave of Covid-19 infection, then it is vital that we have widespread accessible community testing, and the ability to rapidly trace and isolate contacts."

Abstract: We describe what we believe is the first instance of complete COVID-19 testing of all passengers and crew on an isolated cruise ship during the current COVID-19 pandemic. Of the 217 passengers and crew on board, 128 tested positive for COVID-19 on reverse transcription–PCR (59%). Of the COVID19-positive patients, 19% (24) were symptomatic; 6.2% (8) required medical evacuation; 3.1% (4) were intubated and ventilated; and the mortality was 0.8% (1). The majority of COVID-19-positive patients were asymptomatic (81%, 104 patients). We conclude 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.