WHO clarifies comments on asymptomatic spread of coronaviruses: “There are many unknowns”


Maria Van Kerkhove, WHO technical manager for the coronavirus response and head of her emerging diseases and zoonoses unit, said at a press conference in Geneva on Monday that “it still seems rare that a person asymptomatic actually transmits to a secondary individual ”.But on Tuesday during the question and answer session, she said “she is a major unknown.”

“The majority of the transmission we know is that people with symptoms spread the virus to other people through infectious droplets – but there is a subset of people who do not develop symptoms and who understand really how many people do have symptoms, we don’t have an answer about it yet, “said Van Kerkhove.

“We know that some people who are asymptomatic or have no symptoms can transmit the virus,” she said. “So what we need to understand better is how many people in the population are symptom-free and separately how many of these people continue to transmit to others. ”

Van Kerkhove said on Monday that what appear to be asymptomatic Covid-19 cases often turn out to be mild.

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“When we go back and say how many of them were really asymptomatic, we find that many have a really mild illness,” said Van Kerkhove on Monday.

“These are not symptoms of Covid in quotation marks – which means that they may not have developed a fever yet, that they may not have coughed significantly, or that they ‘have no shortness of breath – but some may have a mild illness,’ Van Kerkhove told me. “Having said that, we know there can be some really asymptomatic people. ”

Van Kerkhove added that she was referring to reports from WHO member states when she made her comments on Monday.

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“What I was referring to yesterday at the press conference was very few studies – about two or three published studies that were actually trying to follow asymptomatic cases, so that infected people over the time, then looked at all of their contacts and see how many more people were infected, “said Van Kerkhove.

“And it’s a very small subset of studies. So I was answering a question at the press conference. I was not stating a WHO policy or something like that, “she said. “Because it’s a major unknown, because there are so many unknowns around it, some modeling groups have tried to estimate what proportion of asymptomatic people can transmit. “

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Dr. Mike Ryan, Executive Director of the WHO Health Emergencies Program, also said in the live question and answer session that there is still much to learn about the possible asymptomatic spread of the coronavirus.

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“Regardless of the proportion of diseases transmitted by asymptomatic individuals, as Maria said, this is unknown,” said Ryan.

“I am absolutely convinced that this is happening. The question is how much, ”he said. “There is a lot to answer about this. There are many unknown things. “

“Trying to cut very fine salami”

Whether a person is presymptomatic or just has very mild symptoms does not matter to the person at the receiving end of the transmission, said Dr. William Schaffner, professor at Vanderbilt University and longtime CDC advisor.

“I thought they were getting very chicky and trying to slice the salami very thinly,” said Schaffner of the WHO comments on asymptomatic spread on Monday.

“You can be vertical and feel 100% or almost 100% and go about your daily business without knowing that you are infected and perfectly capable of transmitting the virus,” said Schaffner. “How can we prevent the spread of the virus by these people who do their full range of normal activities? The answer is social distancing and wearing masks and good hand hygiene and staying away from crowds. This is the formula. ”

Liam Smeeth, professor of clinical epidemiology at the London School of Hygiene and Tropical Medicine, said he was “somewhat surprised” by Van Kerkhove’s original comments.

“It goes against my impressions of science so far that suggest that asymptomatic people (people who never have symptoms) and pre-symptomatic people are an important source of infection for others,” said Smeeth in a written statement distributed by Science Media, based in the United Kingdom. Center Tuesday.

“This is the main basis for steps such as self-isolation and foreclosure – steps we know from yesterday’s two Nature journals have massively reduced the number of people infected and prevented millions of dead in the world, “he said. He was referring to research that estimates that closures until the beginning of April have prevented more than 500 million coronavirus infections in six countries.

“There is scientific uncertainty, but asymptomatic infection could account for about 30% to 50% of cases,” said Smeeth. “The best scientific studies to date suggest that up to half of the cases have been infected by asymptomatic or pre-symptomatic people. “


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