Up to 40% of coronavirus cases are spread by people without symptoms, WHO warns

George Martin
·3-min read
A TV grab taken from a video released by the World Health Organization (WHO) shows WHO Technical Lead Maria Van Kerkhove attending a virtual news briefing on COVID-19 (novel coronavirus) from the WHO headquarters in Geneva on April 6, 2020. - The WHO said on April 6, 2020 that facemasks could be justified in areas where hand-washing and physical distancing were difficult, as it teamed up with Lady Gaga to launch a giant coronavirus awareness concert. (Photo by - / AFP) (Photo by -/AFP via Getty Images)
WHO Technical Lead Maria Van Kerkhove said the disease had been able to spread rapidly through those who don't display symptoms. (Getty)

A World Health Organization expert has warned that 40% of coronavirus cases may have been spread by those who are asymptomatic.

Maria van Kerkhove, an epidemiologist and the WHO's technical lead on the pandemic, told a briefing on Tuesday that people with coronavirus are most infectious when they first show symptoms.

"The majority of transmission that we know about is that people who have symptoms transmit the virus to other people through infectious droplets, but there are a subset of people who don't develop symptoms, and to truly understand how many people don't have symptoms, we don't actually have that answer yet,” Dr Kerkhove said.

"There are some estimates that suggest that anywhere between 6% of the population and 41% of the population may be infected but not have symptoms, with a point estimate of around 16%.

BRIGHTON, UNITED KINGDOM - JUNE 09: Social distancing reminders are on display in preparation for the opening of non essential shops, on June 09, 2020 in Brighton, United Kingdom. As the British government further relaxes Covid-19 lockdown measures in England, this week sees preparations being made to open non-essential stores and Transport for London handing out face masks to commuters. International travelers arriving in the UK will face a 14-day quarantine period. (Photo by Mike Hewitt/Getty Images)
Social distancing reminders on display in Brighton. (Getty)

"We do know that some people who are asymptomatic or some people who don't have symptoms can transmit the virus on.

"And so what we need to better understand is how many of the people in the population don't have symptoms and, separately, how many of those individuals go on to transmit to others."

Dr Kerkhove said some modelling groups have been trying to estimate "what is the proportion of asymptomatic people that may transmit".

Read more: WHO says virus 'worsening' globally

She continued: "And these are estimates and there is a big range from the different models, depending on how the models are done, where they're done, from which country.

"But some estimates of around 40% of transmission may be due to asymptomatic. But those are from models."

Elswhere in the press conference Dr Kerkhove sought to clarify comments she made on Monday in which she suggested the transmission of coronavirus through people not displaying symptoms was "very rare".

She told the previous WHO briefing: "We have a number of reports from countries who are doing very detailed contact tracing.

"They're following asymptomatic cases, they're following contacts and they're not finding secondary transmission onward. It's very rare."

But on Tuesday she explained she had been referring to "some two or three studies".

She added: "I was responding to a question at the press conference. I wasn't stating a policy of WHO or anything like that. I was just trying to articulate what we know.

"And in that I used the phrase 'very rare', and I think that that's a misunderstanding to state that asymptomatic transmission globally is very rare. What I was referring to was a subset of studies."

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