UPDATE (at bottom)
Q: I thought that is why we have to be so careful; this is so confusing!
A: Take a deep breath—despite the click-bait headlines coming out of yesterday’s rather unclear statement, nothing dramatic has changed about what we know about COVID-19 spread.
The important distinction is between TRULY asymptomatic people who NEVER show symptoms, versus pre-symptomatic people who we know can transmit days before showing symptoms. Indeed, the highest viral loads seem to occur just prior to the onset of symptoms relative to later in the disease course.
Also, symptoms are not always recognized or reported. Some “asymptomatic” cases may actually have a mild fever that goes unrecognized or other symptoms easily missed.
While the % of TRULY asymptomatic cases vs. pre-symptomatic or mild symptoms is difficult to pin down, the WHO itself provides a “pooled estimate” of current research of 16%. While it is likely true that people with NO symptoms are less likely to transmit than those with mild or pre-symptomatic infection, this group (so far) makes up a small proportion of infections AND still *can* transmit, even if less likely.
BOTTOM LINE: This all means that pre-symptomatic and mildly symptomatic cases remain an important risk for transmission, and social distancing, masking, and the other precautions being promoted are still the best defense.
Even the professionals at WHO don’t always communicate complicated science as clearly as possible, and the media certainly runs with any story that sounds dramatic. The full report to which the WHO comments refer can be found here.
This article from March is still spot on in covering many of these transmissibility issues.
And this study in Nature estimates that 44% of onward transmission in China occurred from cases before their symptoms appeared (“pre-symptomatic transmission”).
UPDATED:
The WHO has acknowledged the confusion of their statement yesterday: “Dr. Maria Van Kerkhove, who made the original comment at a W.H.O. briefing on Monday, said on Tuesday that it was based on just two or three studies and that it was a “misunderstanding” to say asymptomatic transmission is rare globally.”
Link to original FB post
