WHO: approximately 40% of Covid-19 cases may be asymptomatic and it is not known how infectious they are

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According to an expert from the World Health Organization (WHO), up to 41% of people infected with a coronavirus can be asymptomatic, but it is still not known how infectious they are.

WHO technical manager Covid-19 Maria Van Kerkhove said in a briefing Tuesday that some 40% of coronavirus transmission may be due to asymptomatic cases, according to some estimates.

Dr. Kerkhove sought to clarify the comments she made on Monday when she suggested that it was “very rare” to see asymptomatic people spread the virus.

She said in the previous WHO briefing: “We have a number of country reports that are doing very detailed contact tracing.

“They follow asymptomatic cases, they follow contacts and they do not find secondary transmission. It’s very rare. “

But on Tuesday, she said she was referring to “two or three studies”.

She added, “I was answering a question at the press conference. I was not saying a WHO policy or something like that. I was just trying to articulate what we know.

“And in that, I used the expression” very rare “, and I think it is a misunderstanding to say that asymptomatic transmission in the world is very rare. I was referring to a subset of studies. ”

She said WHO has also relied on unpublished data provided by its member states.

Responding to Dr. Kerkhove’s “very rare” comment, Liam Smeeth, professor of clinical epidemiology at the London School of Hygiene and Tropical Medicine, said: “I was very surprised by the WHO statement, and I do not haven’t seen the data on which the statement is based. sure.

“This goes against my impressions of science so far which suggest that asymptomatic people – who never show symptoms – and pre-symptomatic people are an important source of infection for others.

“This is the main basis for steps such as self-isolation and locking.”

Dr. Kerkhove also said on Tuesday: “The majority of the transmission we know is that people who have symptoms spread the virus to other people through infectious droplets, but there is a subset of people who do not develop symptoms.

“To really understand how many people have no symptoms, we don’t have this answer yet.

“According to some estimates, between 6% and 41% of the population could be infected but show no symptoms, with a point estimate of around 16%.

“We know that some people who are asymptomatic or have no symptoms can transmit the virus.

“And so what we need to understand better is how many people in the population do not have symptoms and separately how many of these people continue to transmit to others. ”

Dr. Kerkhove said that some modeling groups are trying to estimate “what proportion of asymptomatic people are likely to transmit.”

She continued: “And these are estimates and there are a wide range of different models, depending on how the models are made, where they are made, from which country.

“But some estimates of around 40% of transmission may be due to asymptomatic symptoms. But they are models. ”

Keith Neal, Emeritus Professor of Epidemiology of Infectious Diseases at the University of Nottingham, said: “The role played by asymptomatic transmission in the total number of new infections remains uncertain, but symptomatic people are responsible for most of the news Covid infections. -19.

“This reinforces the importance for anyone who has any of the symptoms of Covid-19 to organize a test for themselves as soon as possible and to isolate them until they get the test result . “

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