More than 80% of the passengers and crew infected with coronavirus on an expedition cruise ship did not show any symptoms, raising questions about the true prevalence of "silent" infections, according to a peer-reviewed study published Wednesday in Thorax.
The three researchers said their findings emphasize the need for accurate global data on the number of people, both symptomatic and asymptomatic, who have been infected with the coronavirus.
"It is difficult to find a reliable estimate of the number of COVID positive patients who have no symptoms," Alan Smyth, professor of child health at the University of Nottingham and joint editor-in-chief of Thorax, said in a statement. "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."
It remains unclear what a wider prevalence of infection would mean. The World Health Organization cautioned earlier Wednesday that it's unknown whether people who have been infected with the coronavirus are at risk of becoming infected again.
Researchers have tried to determine the number of asymptomatic Covid-19 patients by conducting studies with antibody tests, which detect whether someone has previously been infected by the virus. However, such tests have been marred by accuracy concerns and it remains unclear how many asymptomatic carriers there are and how infectious they are.
The researchers' findings come from observations aboard a 21-day expedition cruise to the Antarctic. All three researchers said they were aboard the ship, which set sail from Argentina in mid-March, after the WHO had declared the coronavirus a global pandemic. The researchers did not declare any funding for the study.
After eight days aboard the ship, the first case of fever was reported, the researchers said. They added that the ship immediately adopted preventive measures, including confining passengers to their cabins, halting most daily services and requiring crew members to wear protective equipment.
Of the 217 passengers and crew who remained on the ship for the entirety of the voyage, 128 tested positive for the coronavirus, the researchers said. They added that of those who tested positive, 24 exhibited symptoms and 104, or 81%, did not.
The researchers did not specify what test was used but noted that it had a high rate of false negatives, which might explain 10 situations in which passengers who shared the same cabin tested differently.
The researchers said their findings are especially significant for the cruise industry, which has been brought to a standstill by the coronavirus pandemic. Earlier this year, as the virus emerged, cruise ships in Japan and elsewhere became the sites of major outbreaks that led to onboard deaths and crew members quarantined at sea for weeks.
The industry is vying to return to service, led by the largest cruise operator in the world, Carnival Corp., which is slated to resume sailing on Aug. 1.
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