'We don’t actually have that answer yet,' WHO says on asymptomatic transmission of COVID-19

Passengers wearing face masks travel on a Piccadilly Line underground train in London, March 20, 2020 (file).
Passengers wearing face masks travel on a Piccadilly Line underground train in London, March 20, 2020 (file). Copyright AP Photo/Matt Dunham
Copyright AP Photo/Matt Dunham
By Natalie Huet with AP
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"We don’t actually have that answer yet:" a top WHO official says it’s not known how frequently people without symptoms of COVID-19 pass the disease on to others – a day after appearing to suggest that such spread is "very rare."


A top World Health Organization official has walked back comments that appeared to suggest it’s "very rare" for people with no symptoms of COVID-19 to pass the disease on to others.

Maria Van Kerkhove, the WHO’s technical lead on the COVID-19 pandemic, clarified that the actual rates of so-called asymptomatic transmission are not yet known.

"The majority of transmission that we know about is that people who have symptoms transmit the virus to other people through infectious droplets," Van Kerkhove said during a live Q&A session on social media on Tuesday.

"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."

Some estimates suggest anywhere between six and 41 per cent of the population may be infected but not have symptoms, she added.

Her earlier comments had drawn strong criticism from public health experts, many of whom have recommended the use of face coverings in public to try to prevent people who are unknowingly infected from spreading the virus.

The confusion appears to stem in part from what asymptomatic infection is and how the general public might understand it. Sometimes the word is used to describe people who haven’t started showing symptoms but eventually will – in that case, we should rather call them presymptomatic – and research has shown they can actually be contagious before they start feeling ill.

On Monday, Van Kerkhove noted that when cases are initially reported as asymptomatic, "we find out that many have really mild disease."

Some people are truly asymptomatic despite a positive coronavirus test, but contact tracing appears to show they rarely transmit the virus onwards, she added.

"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 and much of that is not published in the literature," she said.

"We are constantly looking at this data and we're trying to get more information from countries to truly answer this question. It still appears to be rare that an asymptomatic individual actually transmits onward," she added, stressing that the focus should be on tracking and isolating symptomatic cases.


On Tuesday, Van Kerkhove clarified that when she talked about "rare" asymptomatic transmission, she was referring to the very limited data that followed such cases.

"What I was referring to yesterday were very few studies, some two or three studies that have been published, that actually try to follow asymptomatic cases," she said.

"That's a very small subset of studies," she continued. "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."

To some, it came across as if the WHO was suggesting asymptomatic people played only a very small part in spreading the virus – even as some recent studies have estimated that people not showing symptoms (whether they’re truly asymptomatic or simply presymptomatic) could account for nearly half of the spread.

"I was surprised by the conviction of that statement because there have clearly been people who have transmitted the infection before they go on to develop symptoms,'' said Keith Neal, an infectious diseases expert at the University of Nottingham who has advised the U.K. government on outbreak control.

Can people who don't feel sick spread the disease?

It seems, but we don't know yet to what extent exactly. How easily the coronavirus spreads in different circumstances is not well understood, and is still being studied.

The WHO has maintained for months that the vast majority of COVID-19 spread is from people with symptoms like a fever or cough, and that transmission from people who don't feel sick is not thought to be a major driver of the disease.


WHO's emergencies chief, Dr. Michael Ryan, said "both symptomatic and asymptomatic individuals are part of the transmission cycle" but that it was unclear how much each contributed to the spread of the virus.

Does wearing a mask help?

Wearing a mask might not protect you from getting sick as your eyes are likely still exposed, but health experts think that it may prevent you from spreading the disease unknowingly by acting as a physical barrier.

COVID-19 is spread via respiratory droplets, so wearing a mask will stop those droplets from reaching others.

While most spread is thought to happen by coughing and sneezing, Ryan said there is some suggestion that even acts like singing or shouting could spread the virus from people who don't yet show symptoms.

Why don't we know for sure?

It's complicated, especially since the virus was only identified in late December. Some scientists said WHO's distinction between people who are truly asymptomatic (those who are infected by COVID-19 but never show symptoms) and those who are pre-symptomatic (and develop symptoms later) explains part of the confusion.


While truly asymptomatic people are likely not responsible for significant virus spread, several studies have documented people spreading the disease before they get sick – and some experts say recognising and stopping this kind of transmission is critical to controlling the pandemic.

Detailed studies and testing of people who test positive for the coronavirus but don't show symptoms to determine if they spread the disease are needed – and few have been done so far.

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