A top WHO official on Tuesday clarified her remarks that transmission of the new coronavirus from asymptomatic carriers was "very rare", citing a "misunderstanding".
Maria Van Kerkhove, the World Health Organisation's Covid-19 technical lead, had said that on the basis of studies carried out in several countries, transmission of the virus by an asymptomatic person seemed "very rare".
"We have a number of reports from countries who are doing very detailed contact tracing. They're following asymptomatic cases, they're following contacts and they're not finding secondary transmission onward. It's very rare," she told a virtual press conference on Monday.
Her remarks, which were widely relayed on social media networks, sparked a reaction from part of the scientific community.
"Contrary to what the WHO announced, it is not scientifically possible to affirm that asymptomatic carriers of SARS-CoV-2 are not very infectious," professor Gilbert Deray of the Pitie-Salpetriere hospital in Paris said on Twitter.
Liam Smeeth, a clinical epidemiology professor at the London School of Hygiene and Tropical Medicine, said he was "quite surprised".
"There remains scientific uncertainty, but asymptomatic infection could be around 30 percent to 50 percent of cases. The best scientific studies to date suggest that up to half of cases became infected from asymptomatic or pre-symptomatic people," he said.
Van Kerkhove later posted on Twitter a WHO summary on transmission.
"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," it said.
During a discussion rebroadcast Tuesday on the WHO's Twitter account, Van Kerkhove said she wanted to clarify a misunderstanding.
"I was referring to very few studies, some two or three", and answering a question.
"I was not stating a policy of WHO," she said.
"I used the phrase 'very rare', and I think that is a misunderstanding to state that asymptomatic transmission globally is very rare. What I was referring to was the subset of studies," she added.