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Infectious Diseases

Updated 03 June 2020

'Silent' Covid-19 more widespread than thought, study suggests

The finding that many more people than previously believed may have been infected with SARS-CoV-2 could be important as lockdowns are eased and accurate numbers are needed.

  • Number of those who are infected, but without symptoms may be higher than thought
  • Researchers looked at how the virus spread through a cruise ship
  • Majority of those who tested positive on the vessel had no symptoms 


A new cruise ship study suggests that the number of people who are infected with the new coronavirus but have no symptoms may be much higher than believed.

More than 80% of those who tested positive for the infection had no symptoms, according to the study published online on 27 May in the journal Thorax.

The findings could be important as lockdown restrictions begin to be eased, and they highlight the need for accurate data on how many people worldwide have been infected, journal joint editor-in-chief Alan Smyth said in a linked blog.

The study focused on a ship with 128 passengers and 95 crew that left Argentina in mid-March for a planned 21-day cruise of the Antarctic. The voyage began after the World Health Organization (WHO) had declared the new coronavirus a global pandemic.

Medical evacuation

Passengers who in the previous three weeks had been in countries where coronavirus infection rates were high were not allowed to board. All passengers who were allowed to board had their temperature taken before embarkation, and the ship had numerous hand sanitising stations, particularly in the dining room.

After the first case of fever was reported on day eight of the voyage, infection control measures were immediately implemented, including confining passengers to their cabins and the use of personal protective equipment by any crew member in contact with sick passengers.

The ship arrived in Uruguay on day 13, where eight passengers and crew eventually required medical evacuation to hospital for Covid-19-related respiratory failure.

On day 20, all remaining 217 passengers and crew were swab tested for the new coronavirus, and more than half (59%) tested positive. However, 81% of those who tested positive had no symptoms.

The findings suggest that coronavirus infection rates on cruise ships are likely to be "significantly underestimated", and that passengers should be monitored after leaving the ships to reduce the risk of community spread of the virus, according to the researchers.

Many more infections than suspected

The study authors also said that the potentially high rate of false-negative results obtained with current swab tests suggests the need for secondary testing.

"It is difficult to find a reliable estimate of the number of Covid-positive patients who have no symptoms," but the rate of 1% suggested by the WHO in early March is far lower than that on the cruise ship in this study, Smyth said in his blog.

"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 noted.

Whether or not people who have been infected are immune to the new coronavirus, the findings emphasise an urgent need for accurate global data on how many people have been infected, he concluded.

Image credit: Getty Images