This post is going to mislead many people and is based on flawed evidence due to how it was collected. Sadly it's not the 1st time WHO is putting out poor information. I'll explain.
Early in epidemic there were Qs whether people w/out symptoms could spread the disease. It is critically important and fundamentally determines whether quarantining based on symptoms will be effective. See fantastic paper on this by @ChristoPhraser pnas.org/content/101/16…
Abundant data now shows that transmission from people w/out symptoms makes up a huge fraction (40-50%) of transmission. But a key question has been whether transmission is from pre-symptomatic people who develop symptoms later or truly asymptomatic people who never do.
This Q is not answered and some studies have shown evidence in support of asymptomatic cases being equally infectious as symptomatic ones whereas other show milder cases to be less infectious. Here's 1 of each:
medrxiv.org/content/10.110…
bmj.com/content/bmj/36…
medrxiv.org/content/10.110…
bmj.com/content/bmj/36…
The bigger problem is that contact tracing studies are flawed due to the delays b/w infection and testing. When a person is tested they usually have had at least mild symptoms for several days and were infected ~5.5 days before that. medrxiv.org/content/10.110…
As a result, the person that infected them was infectious 8-11d ago. If that person was asymptomatic there's a moderate chance they would test negative when traced (unfortunately we don't know the exact probability of that: ).
Data from symptomatic people by @bennyborremans suggest it could be high- 10d after symptom onset 30% of infected people test negative by swab. medrxiv.org/content/10.110… 
It *could* be higher for asymptomatic people & even if not, one could mis-attribute transmission due to delay. For symptomatic people you'd attribute them to be infector based on symptoms onset date (all studies I know of use earlier onset date to ID infector vs infectee).
Given uncertainty in infectiousness and bias in contact tracing methodology, there is insufficient for @WHO to make this claim for asymptomatic transmission and given the confusion b/w pre-symptomatic and truly asymptomatic it's a misleading PR disaster.
insufficient *evidence* to make for @WHO to make this claim. Sorry for the important typo.
