Maria Van Kerkhove, infectious disease epidemiologist and Covid-19 Technical Lead at the WHO, was forced to clarify the comments after her initial statement provoked a backlash from medical professionals worldwide, and she stood accused of spreading unhelpful information among users on Twitter.
At a virtual WHO press conference, Ms Van Kerkhove had originally said: “It seems rare that an asymptomatic person would spread the virus.”
SARS-CoV-2 is the coronavirus that causes Covid-19.
The statement was roundly contradicted by medical professionals online, including Professor Gilbert Deray, at the Pitié-Salpêtrière hospital in Paris.
Writing on Twitter, he said: “Contrary to what the WHO has announced, it is not scientifically possible to confirm whether asymptomatic carriers of SARS-Cov-2 are low spreaders.”
And in response to a question under the Tweet asking if there were "studies saying the opposite", Professor Deray simply said: "Yes."
Similarly, Professor Liam Smeeth, of the London School of Hygiene and Tropical Medicine, said he was “surprised” by the WHO’s statement.
He told newswire Reuters: “It goes against my impressions from the science so far, which suggests [that] asymptomatic people – who never get symptoms – and pre-symptomatic people are an important source of infection to others.”
In a further statement, Professor Smeeth said: "There remains scientific uncertainty, but asymptomatic infection could be around 30%-50% of cases. The best scientific studies to date suggest that up to half of cases became infected from asymptomatic or pre-symptomatic people.”
Comment clarification
In light of the reactions, Ms Van Kerkhove later sought to “clarify” her comments, and said there had been “a misunderstanding”.
Quoting from “a summary of transmission of Covid-19 incl. symptomatic, pre-symptomatic and asymptomatic transmission” by the WHO, she Tweeted:
“Comprehensive studies on transmission from asymptomatic individuals are difficult to conduct, but 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.
“In these data, it is important to break down truly asymptomatic vs pre-symptomatic vs mildly symptomatic; also to note that the % reported or estimated to be ‘asymptomatic’ is not the same as the % that are asymptomatic that actually transmit.”
She added that the initial statement was “in reference to a very small number of studies, two or three”, and had been made in a reply to a question from a journalist, rather than an official statement on the formal position of the WHO.
She added: “I used the phrase ‘very rare’, but it is a misunderstanding to say that the spread by asymptomatic carriers is very rare; I was making reference to a small group of studies.”
Asymptomatic carriers: Early studies
Asymptomatic carriers are people who have the virus, and can pass it on to others, but do not have any symptoms themselves (or they only have very mild, almost imperceptible symptoms).
A study by the infectious diseases department at the Zhongnan hospital in China studied whether “a profile” of an asymptomatic carrier could be created and understood.
In the study, researchers performed clinical exams of 78 patients that had been exposed to the virus, and found that 33% were asymptomatic. This equates to 42.3% of the participants.
The researchers found that even if someone felt no symptoms of the virus, medical scanners were able to detect traces of it in their body. They also found that the recovery time for an asymptomatic patient was nine days on average, versus 15 days for someone who felt ill.
An asymptomatic person was found to have less apparent immune system damage due to the virus compared to someone who had felt unwell.
So far, the WHO has said that the risk of contagion exists for 14 days, from the time that a person is infected and/or that symptoms appear. Someone infected with the virus will, on average, pass it to three further people, if they do not take precautions.
These include self-isolating, wearing a mask, and regularly washing hands.
An asymptomatic person can infect anything they touch and may therefore become an unwitting super-spreader, because they may not know that they have the virus and so are unlikely to take precautions.
Another Chinese study, published in May by American health authorities, showed that two infected-but-asymptomatic Chinese students who had been placed in quarantine in a hotel had contaminated 36% of 36 objects sampled within 24 hours.
These included their bed sheets and pillows, which were especially contaminated.
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