A better understanding is needed to explain how coronavirus spreads between people, a World Health Organisation expert has said, after a previous comment that asymptomatic transmission was “very rare” provoked confusion and surprise.
In a briefing on Tuesday, Dr Maria Van Kerkhove, the WHO’s Covid-19 technical lead, sought to clarify her earlier remarks, insisting there remains “a big open question” over the role played by symptomless people in passing on the virus.
She stressed that her previous observation, made on Monday, was based on a small set of studies provided by countries “who are doing very detailed contact tracing”.
“I wasn’t stating a policy of WHO or anything like that. I was just trying to articulate what we know,” Dr Kerkhove said.
“I used the phrase ‘very rare’ and I think that it’s a misunderstanding to state the asymptomatic transmission globally is very rare. What I was referring to was a subset of studies. I was also referring to some data that isn’t published.”
Although a large proportion of people with no symptoms test positive, it is not known how many of these individuals go on to infect others. “We don’t have the answer yet,” she added.
Liam Smeeth, professor of Clinical Epidemiology at the London School of Hygiene and Tropical Medicine, said he had been “quite surprised” by Dr Kerkhove’s initial comments.
“It goes against my impressions from the science so far that suggest asymptomatic and pre-symptomatic people are an important source of infection to others,” he said.
Keith Neal, an infectious diseases expert at the University of Nottingham, said: “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.”
Some modelling groups estimate around 40 per cent of coronavirus transmission may be due to asymptomatic cases, while research suggests the proportion of people with Covid-19 who do not display symptoms could be anywhere between 6 per cent and 41 per cent, Dr Kerkhove added in her Tuesday briefing.
The director of the WHO’s health emergencies programme, Dr Michael Ryan, said he was “absolutely convinced” asymptomatic transmission was occurring, “the question is how much”.
Dr Kerkhove explained there are three types of categories involved in the transmission of Covid-19: those with symptoms, who are considered the most infectious: those who test positive and later go on to develop symptoms (pre-symptomatic): and those with the virus who do not develop any symptoms (asymptomatic).
However, scientists remain unsure how long people are infectious for prior to the development of symptoms and a clear illness. “It is not only who is transmitting to others, but when,” she said. “The data is very preliminary.” Dr Kerkhove added: “It appears that people have more virus in their body around the time they develop symptoms.”
“With Covid-19, the infectious pathogen is in the upper airway where viral loads are peaking just as you are starting to feel sick,” said Dr Ryan.
The recent communications debacle has also brought into focus the WHO’s recent change to its longstanding mask advice.
On Friday, WHO revised its mask policy, recommending that people wear fabric masks if they could not maintain social distancing, if they were over age 60, or had underlying medical conditions. Part of the reasoning, WHO officials said, was to account for the possibility that transmission could occur from people who had the disease but were not yet symptomatic.
“The available evidence from contact tracing reported by member states suggests that asymptomatically-infected individuals are much less likely to transmit the virus than those who develop symptoms,” the WHO said in guidance on mask wearing.
