Ashish K. Jha+ Your Authors @ashishkjha Physician, researcher, and advocate for the notion that an ounce of data is worth a thousand pounds of opinion. Director @harvardgh. Soon, Dean @Brown_SPH Jun. 08, 2020 1 min read + Your Authors

This from @WHO is getting a lot of attention and creating confusion.

I want to quickly share what I understand about this.

Bottom line question:

Are infected people without symptoms an important cause of spread?

My best guess: yes.

A thread

First, about 20% of people who are infected likely never develop any symptoms.

They are truly asymptomatic.

Some variation in estimates on this. Lots of data points but here's a preprint 


So what about the other 80% of people who do have symptoms?

Many of them are shedding virus BEFORE they develop symptoms 

Technically, these folks are PRE-symptomatic, not asymptomatic

But they are asymptomatic at the time they are shedding virus


Some modeling studies suggest 40-60% of spread is from people when they didn’t have symptoms.

Here are a few refs: 


So – it might be @WHO is drawing a distinction between asymptomatic spread and pre-symptomatic spread.

And it may be there isn't a lot of asymptomatic spread but plenty of pre-symptomatic spread.

Would be helpful to get the full report that they are referencing.


Both asymptomatic AND pre-symptomatic spread huge problem for controlling disease

Because folks shedding virus while asymptomatic

Pre-symptomatic has one advantage: you can use contact tracing to find folks they infected

But that doesn’t help prevent presymptomatic spread


.@WHO communication here not stellar

If folks without symptoms truly "very rarely" spread virus, would be huge.

But such a statement by @WHO should be accompanied by data.

Asymptomatic spread is Achille's heal of this outbreak

Would love to be wrong. Need to see data


More on @WHO comment by @mvankerkhove

As I read her follow-on tweets, best guess is she really is differentiating asymptomatic vs pre-symptomatic

Two key points:

1. People without symptoms definitely spread disease (so wear a mask)

2. @WHO should be clearer in communication

You can follow @ashishkjha.


Tip: mention @threader_app on a Twitter thread with the keyword “compile” to get a link to it.

Enjoy Threader? Sign up.

Since you’re here...

... we’re asking visitors like you to make a contribution to support this independent project. In these uncertain times, access to information is vital. Threader gets 1,000,000+ visits a month and our iOS Twitter client was featured as an App of the Day by Apple. Your financial support will help two developers to keep working on this app. Everyone’s contribution, big or small, is so valuable. Support Threader by becoming premium or by donating on PayPal. Thank you.

Follow Threader