Cruise ship research shows prevalence of 'silent' infection

The prevalence of ‘silent’ symptomless Covid-19 infection may be much higher than thought, reveals a study charting the enforced isolation of cruise ship passengers during the pandemic.

More than eight out of 10 of passengers and crew who tested positive for the infection had no symptoms.

This has implications for the easing of lockdown restrictions, says Professor Alan Smyth, joint editor in chief of the journal Thorax, in a linked blog.

And the findings emphasise the pressing need for accurate global data on how many people have been infected.

The researchers, all of whom were on board the vessel which has not been named for patient confidentiality, describe events on an expedition cruise ship carrying 128 passengers and 95 crew.

The ship departed from Ushuaia, Argentina, for a planned 21-day cruise of the Antarctic, taking a similar route to that of Ernest Shackleton in 1915-17.

It set sail in mid-March after the World Health Organisation had declared Covid-19 a global pandemic.

Passengers who, in the previous three weeks, had passed through countries where Covid-19 infection rates were already high, were not allowed to board.

And everyone’s temperature was taken before embarkation.

Hand sanitising stations were plentiful aboard the ship, particularly in the dining room.

The first case of fever was reported on day 8, prompting the immediate adoption of infection control measures.

This included confining passengers to their cabins, stopping daily servicing, apart from the delivery of meals, and the wearing of personal protective equipment for any crew member in contact with sick passengers.

As Argentina had closed its borders and permission to disembark at Port Stanley on the Falkland Islands was refused the ship sailed to Montevideo, Uruguay, arriving on day 13.

Eight passengers and crew eventually required medical evacuation to hospital at this point for respiratory failure.

On day 20 all the remaining 217 passengers and crew were swab tested for coronavirus.

More than half (128; 59 per cent) tested positive.

In 10 instances, two passengers sharing the same cabin did not have the same test result, possibly because the current swab test returns a substantial number of false negative results, say the authors.

Of those testing positive, 24 (19 per cent) had symptoms, but 108 (81 per cent) did not.

The ship had no contact with other people for 28 days after its departure, so was the equivalent of a hermetically sealed environment.

The study authors conclude that the prevalence of Covid-19 infection on cruise ships is likely to be “significantly underestimated,” prompting them to recommend that passengers should be monitored after disembarkation to ward off potential community spread of the virus.

And the potentially high rate of false negative results obtained with the current swab tests suggests that secondary testing is warranted, they add.

In a linked blog, Professor Smyth acknowledges: “It is difficult to find a reliable estimate of the number of Covid positive patients who have no symptoms.”

But the figure of one per cent suggested by the WHO in early March falls far short of that found on the cruise ship, he points out.

“As countries progress out of lockdown, a high proportion of infected, but asymptomatic, individuals may mean that a much higher percentage of the population than expected may have been infected with Covid-19,” he suggests.

But whether or not those who have been infected are immune, the findings emphasise a pressing need for accurate global data on how many people have been infected, he concludes.

On 20 February, the World Health Organisation announced that more than half the known cases of covid-19 in the world outside China were on a single ship: the Diamond Princess.

A 16-year-old luxury vessel that cost half a billion dollars to build was stuck in quarantine in Japan with more than 3700 people on board. Hundreds of them had become sick and were confined to their cabins.

The situation was widely lamented. A Japanese epidemiologist described conditions aboard the Diamond Princess as “completely chaotic.

The US Centers for Disease Control and Prevention questioned the efficacy of the onboard lockdown, and infectious disease expert Anthony Fauci, who is advising the White House on thepandemic, said the vessel’s quarantine process had “failed”.

Cruises around the world were suspended on 14 March, yet many ports refused to let the ships dock long before that, meaning some crews have been stuck at sea for months.

Thorax is one of the world’s leading respiratory journals, publishing clinical and experimental research articles on respiratory medicine, paediatrics, immunology, pharmacology, and surgery.