Coronavirus: Cruise ship outbreak reveals scale of silent carriers of Covid-19

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New research has suggested the number of people who can be infected with coronavirus but show no symptoms could be far higher than previously thought.

According to a study of passengers on a cruise ship affected by Covid-19, more than eight out of 10 tested positive for the virus but had no symptoms.

Professor Alan Smyth, joint editor of the journal Thorax, said the results had implications for easing lockdown restrictions.

He said: “It is difficult to find a reliable estimate of the number of Covid-19-positive patients who have no symptoms.

“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.”

The study was carried out by researchers who were on board the unnamed ship as part of an expedition with 128 passengers and 95 crew.

The ship departed from Argentina for a planned 21-day cruise of the Antarctic in mid-March, after the World Health Organisation had declared Covid-19 a pandemic.

Passengers who had passed through countries where Covid-19 infection rates were already high were not allowed to board and temperature checks were carried out before the ship set sail.

The first case of fever was reported on day eight of the cruise with passengers confined to their cabins. Food was delivered to their rooms and staff wore protective equipment when in contact with sick passengers.

The ship sailed to Uruguay where eight passengers and crew disembarked and were taken to hospital. On day 20 of the cruise, the remaining 217 people were swabbed for the virus with more than half, 128 people, testing positive.

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

The study authors concluded the prevalence of the virus on cruise ships was likely to be “significantly underestimated”, and they warned passengers should be monitored after ships set sail to ward off community spread of the virus.

They added that repeated tests may be necessary to counter false negative results.