'Silent' virus may be more prevalent than we thought

A study charting the enforced isolation of cruise ship passengers during the current pandemic has found that the prevalence of ‘silent’ symptomless COVID-19 infection may be much higher than thought. The study, published in the journal Thorax, reveals that eight out of 10 passengers and crew who tested positive for the infection had no symptoms.

Thorax joint Editor-in-Chief Professor Alan Smyth said that the findings have implications for the easing of lockdown restrictions and the pressing need for accurate global data on how many people have been infected.

The researchers, all of whom were onboard the vessel, 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, setting sail in mid-March after the World Health Organization (WHO) 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. Temperatures were taken before embarkation — hand sanitising stations were plentiful aboard ship, particularly in the dining room.

The first case of fever was reported on day eight, 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, 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, the remaining 217 passengers and crew were swab tested for coronavirus. More than half (128; 59%) tested positive.

In 10 instances, two passengers sharing the same cabin didn’t have the same test result. The study authors suggest that this may be because the current swab test returns a substantial number of false-negative results.

Of those testing positive, 24 (19%) had symptoms, but 108 (81%) didn’t.

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. They add that the potentially high rate of false-negative results obtained with the current swab tests warrants secondary testing.

In a linked blog, Professor Smyth acknowledged that while it is difficult to find a reliable estimate of the number of symptomless COVID-19-positive patients, the figure of 1% suggested by the WHO in early March falls far short of that found on the cruise ship.

“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,” he said.

Image credit: ©stock.adobe.com/au/DOF