Rates of 'symptomless' Covid-19 infections may be higher than previously thought

Thomas SAMSON  AFP

LISTEN MAY 27 2020

Of the 217 passengers and crew on board, 128 tested positive for the virus, according to a study led by Professor Alvin Ing of Macquarie University and published today in the journal Thorax.

Researchers have known for months that asymptomatic transmission of COVID-19 is possible and common, but without population-wide testing, it's been hard to estimate how many people get infected without showing symptoms.

They said strategies are needed to assess and monitor all passengers to prevent community transmission after disembarkation. Asymptomatic cases pose a problem as the patients are tested Covid-19 positive but develop no symptoms such as fever, cough or sore throat.

The (unnamed) cruise ship in the new paper left Argentina in mid-March, with plans to travel around the Antarctic Peninsula and over to South Georgia Island in the south Atlantic ocean.

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

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"Many people still haven't grasped the notion that asymptomatic people can be so common, and they wonder why it is they have to wear the mask when they're feeling well, or why they have to keep doing this social distancing stuff", Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center who was not involved in either study, told the outlet.

It set sail after the global pandemic was declared by the World Health Organisation and two of the study authors happened to be passengers, while a third was the expedition physician.

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

The ship was not allowed to re-dock in Argentina after the country closed its borders, so it continued on to Uruguay, where eight people were evacuated to a hospital.

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

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"It is hard to find a reliable estimate of the number of COVID positive patients who have no symptoms", Professor Smyth said.

With the potentially high rate of false negative results obtained with the current swab tests, they also believe secondary testing is warranted.

"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 explained.

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

In total, just 24 of the 128 Covid-19 positive patients were symptomatic.

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