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Infectious Diseases

Updated 10 June 2020

Top WHO official backtracks on comments that asymptomatic spread of Covid-19 virus is 'rare'

A top WHO official has backtracked on comments that asymptomatic spread of the coronavirus was very rare", saying it was not WHO policy - and she was misunderstood.

  • In a WHO press conference on Monday, a top official said asymptomatic spread of the coronavirus "seemed to be rare"
  • This, she said was based on data from various countries with in-depth contact tracing reports
  • However, on Tuesday WHO clarified their comments in a live video, stating that there’s not enough data to be certain

As the world grapples with the pandemic, social distancing and lockdown measures have widely been implemented on the basis that many carriers might not know that they have the coronavirus, with mild or no symptoms to prompt getting tested.

During a briefing on Monday, a top World Health Organisation official a made a startling statement on asymptomatic carriers of the virus, prompting backlash.

Dr Maria van Kerkhove - head of WHO’s Emerging Diseases and Zoonosis Unit - said that it would seem that it’s rare that asymptomatic people spread Covid-19. 

“We have a number of reports from countries that are doing very detailed contact tracing. They are following asymptomatic cases, they’re following contacts and they’re not finding secondary transmission onward - it’s very rare and not much of that is published in the literature,” said Van Kerkhove in the conference. 

“We are constantly looking at this data and we’re trying to get more information from countries that truly answer this question. It still appears to be rare that an asymptomatic person actually transmits onward.”

Asymptomatic cases are generally identified through contact tracing, as many countries like South Africa require symptoms like fever and a persistent cough to qualify for a Covid-19 test if you haven’t been in direct contact with a positive case. 

Clarity on the comments - there is no certainty

However, on Tuesday in a live online video, Van Kerkhove alongside Dr Mike Ryan - director of WHO’s Health Emergencies Programme, set out to clarify these comments. 

Van Kerkhove said they don’t know yet how many of the population are asymptomatic - could be between 6% and 41% of cases according to estimates. She also based her comments on a small subset of studies that looked at asymptomatic transmission, laid out in the WHO’s guidance on the use of masks published on Friday. 

According to this guidance, one study showed that 14% of 63 asymptomatic patients in China had transmitted the virus, while another showed no secondary transmission. In summary, the WHO concludes that there isn’t enough data on asymptomatic cases to make a final call. 

Van Kerkhove explained she was responding to a question at Monday’s conference and not stating a policy of the WHO. Her comment that asymptomatic transmission was "very rare" was misunderstood and wasn’t a global generalisation, she said - and based her comment on data that isn’t published yet and what WHO receives from their member states. 

“What I didn’t report yesterday, is that because this is a major unknown, because there are so many unknowns around this, some modelling groups have tried to estimate what is the proportion of asymptomatic people that may transmit.”

While these models varied in data from different countries, some models purported that around 40% of transmission may be due to asymptomatic cases, said Van Kerkhove.

Asymptomatic vs presymptomatic

Van Kerkhoven also mentioned that while there is evidence of truly asymptomatic cases, many times those who test positive are presymptomatic and only show symptoms after a few days, or have very mild symptoms. 

There is some debate online on the terminology of asymptomatic and presymptomatic, as they are sometimes interchangeably used in popular media and the range of symptoms have shifted to include other ailments like loss of smell and taste.

She also mentioned a previous study published by the US’s Centres for Disease Control and Prevention (CDC), where cases tracked in Singapore where part of a cluster spread that was the result of presymptomatic transmission. This study was used by the CDC to bolster the need for social distancing and the avoidance of crowds.

In another study published in March, researchers wanted to emphasise the importance of documenting even the mildest, asymptomatic cases, arguing that people who carry the Covid-19 virus without any knowledge thereof can cause a large number of infections as they spread the virus.

But if asymptomatic people don’t get coughs, how do they spread the virus? 

Ryan explained that just talking, exercising, breathing loudly and touching your mouth and then another surface are ways that asymptomatic people can spread the coronavirus. 

Contact tracing of symptomatic cases key

But Van Kerkhove said the real focus should by on following symptomatic cases. 

“If we actually followed all of the symptomatic cases, isolated those cases, follow the contacts and quarantine those contacts... it would be a drastic reduction in transmission. If we could focus on that, I think we would do very well in suppressing transmission.”

Ryan reiterated this sentiment, saying that countries with limited resources have to be tactical when it comes to testing and contact tracing. 

“It’s not easy to stop this virus,” added Ryan. 

Image credit: Getty Images