Coronavirus: Asymptomatic Transmission Still Not Confirmed
Dr Maria Van Kerkhove said on Monday it was "very rare" for asymptomatic people to pass the disease on.
But she has now stressed this observation was based on a relatively small set of studies.
Evidence suggests people with symptoms are most infectious, but the disease can be passed on before they develop.
Although a proportion of people test positive with no symptoms, it is not known how many of these people go on to infect others.
Dr Van Kerkhove said the evidence she had been discussing came from countries that had carried out "detailed contact tracing",
Looking at investigations of clusters of infections from various countries, she said that where an asymptomatic case had been followed up it was "very rare" to find secondary infections among their contacts.
But she it was still a "big open question" as to whether the same was true globally.
The uncertainties involved emphasise the importance of lockdown measures in "massively reduc[ing] the numbers of people infected," said Prof Liam Smeeth, an epidemiologist at the London School of Hygiene and Tropical Medicine.
He said he had been "surprised" by the WHO statement but had not seen the data it was based on.
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".