Asymptomatic patients can spread coronavirus, say WHO – as they backtrack within 24 hours

ASYMPTOMATIC patients can spread coronavirus, the World Health Organisation (WHO) have clarified.

Dr Maria Van Kerkhove, head of the WHO’s emerging diseases and zoonosis unit, caused a stir on Monday by claiming asymptomatic transmission was “very rare”.

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However, WHO have now backtracked and acknowledged that a large percentage of coronavirus infections could be transmitted by people who have the virus but no symptoms.

Van Kerkhove said during a media briefing in Geneva on Monday that “it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual.”

However, on Tuesday, during a social media Q&A, she clarified “this is a major unknown.”

“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 answered yet,” Van Kerkhove said.

We do know that some people who are asymptomatic, or some people who don’t have symptoms, can transmit the virus on

Dr Maria Van Kerkhove

“We do know that some people who are asymptomatic, or some people who don’t have symptoms, can transmit the virus on,” she said.

“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.”

On Monday, Van Kerkhove had said that what appear to be asymptomatic cases of Covid-19 often turn out to be cases of mild disease.

She said: “When we actually go back and we say how many of them were truly asymptomatic, we find out that many have really mild disease.

‘Mild disease’

“They’re not quote-unquote Covid symptoms – meaning they may not have developed fever yet, they may not have had a significant cough, or they may not have shortness of breath — but some may have mild disease.

“Having said that, we do know that there can be people who are truly asymptomatic.”

Van Kerkhove added that her comment about the rarity of asymptomatic spread was based on two or three studies following up the contacts of asymptomatic people, and unpublished data shared by countries or experts with her organisation.

“What I was referring to yesterday in the press conference were very few studies – some two or three studies that had been published that actually try to follow asymptomatic cases, so people who are infected, over time, and then look at all of their contacts and see how many additional people were infected,” Van Kerkhove said.

“And that’s a very small subset of studies. So I was responding to a question at the press conference.

“I wasn’t stating a policy of WHO or anything like 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.”

She said she did not mention the estimates of up to 40 per cent because “those are from models”.

I’m absolutely convinced that asymptomatic spread is occurring. The question is how much

Dr Mike Ryan

Dr Mike Ryan, executive director of the WHO Health Emergencies Programme, also said during the live Q&A that there is still a lot to learn about the possible asymptomatic spread of Covid-19.

He said: “Whatever proportion of disease is transmitting from asymptomatic individuals, as Maria said, that is unknown.

“I’m absolutely convinced that that is occurring. The question is how much.

“There’s much to be answered on this. There’s much that is unknown.”

Prof Liam Smeeth, an epidemiologist at the London School of Hygiene and Tropical Medicine, said that asymptomatic infection “could be around 30 per cent to 50 per cent of cases.”

He added: “The best scientific studies to date suggest that up to half of cases became infected from asymptomatic or pre-symptomatic people.”

And Prof Keith Neal, an epidemiologist at the 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.”

Detailed contact tracing

Prof Babak Javid, a consultant in infectious diseases at Cambridge University Hospitals, said it was important to distinguish between those with very mild or no symptoms.

He explained that those who never develop symptoms are known as “true asymptomatics”, while “presymptomatics” are those who don’t have symptoms at the time of testing positive for the virus but later develop symptoms and “paucisymptomatics” who are those who have atypical or very mild symptoms.

Prof Javid said: “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.

“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.”