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expert reaction to comments from Dr Maria van Kerkhove at yesterday’s WHO press conference that asymptomatic transmission of SARS-CoV-2 is rare

At the World Health Organisation (WHO) press conference on Monday 8th June, Dr Maria van Kerkhove stated that asymptomatic transmission of SARS-CoV-2 is rare.


Tuesday 9th June

Prof Liam Smeeth, Professor of Clinical Epidemiology, London School of Hygiene and Tropical Medicine, said:

“I was quite surprised by the WHO statement, and I have not seen the data the statement is based on.

“It goes against my impressions from the science so far that suggest asymptomatic (people who never get symptoms) and pre-symptomatic people are an important source of infection to others. This is the main basis for steps such as self-isolation and lockdown – steps we know, from yesterday’s two Nature papers have massively reduced the numbers of people infected and have prevented millions of deaths globally.

“What is true is that once we have successfully interrupted community transmission, our main can shift and focus on case finding and testing of symptomatic people and tracing, testing and self-isolation of their contacts.

“There remains scientific uncertainty, but asymptomatic infection could be around 30% to 50% of cases. The best scientific studies to date suggest that up to half of cases became infected from asymptomatic or pre-symptomatic people.”


Prof Keith Neal, Emeritus Professor of the Epidemiology of Infectious Diseases, University of Nottingham, said:

“How much of a role asymptomatic transmission plays in the total number of new infections remains unclear but symptomatic people are responsible for most of the new infections of COVID-19.

“This reinforces the importance of any person who has any of the symptoms of COVID-19 arranges a test for themselves as soon as possible and isolating until they get their test result. 

“Everyone has a role to play in stopping COVID-19.”


Monday 8th June

Prof Babak Javid, Principal Investigator, Tsinghua University School of Medicine, Beijing, and Consultant in Infectious Diseases at Cambridge University Hospitals, said:

“In the WHO’s press conference today, it was suggested that patients with asymptomatic infection rarely transmit Covid to others. Dr van Kerkhove makes the important distinction between true asymptomatics (never develop symptoms), presymptomatics (don’t have symptoms at the time of testing positive for SARS-CoV-2 but later develop symptoms) and paucisymptomatics (have atypical or very mild symptoms). She describes that unpublished data suggest “asymptomatics” (not further clarified) do not transmit infection. This may well be true. Detailed contact tracing from Taiwan as well as the first European transmission chain in Germany suggested that true asymptomatics rarely transmit.

“However, those (and many other) studies have found that paucisymptomatic transmission can occur, and in particular, in the German study, they found that transmission often appeared to occur before or on the day symptoms first appeared (i.e. presymptomatic transmission).

“Without having access to the data Dr van Kerkhove refers to, it is difficult to make any other assessment. I’m sure those data will become publicly available in due course. In the meantime, other data available, from studies in several continents confirming that presymptomatic transmission does occur would suggest that being well does not necessarily mean one cannot transmit SARS-CoV-2. However, the important point is made that some even very mild symptoms that are not ‘typical’ of Covid (i.e. not having a fever or cough) may still represent someone who can be contagious. This has important implications for the track/trace/isolate measures being instituted in many countries.”



The relevant section of the press conference is from 31:42 at this link:


All our previous output on this subject can be seen at this weblink:


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