Dr. Maria Van Kerkhove, head of the WHO’s emerging diseases and zoonosis unit, has begun “walking back” her comment regarding onward transmission of Coronavirus, saying it’s a “really complex question” and there is much we do not know.
Let’s clarify something:
If you read the briefing “Advice on the use of masks in the context of COVID-19″ published by The WHO on June 5th 2020, they state “Viable virus has been isolated from specimens of pre-symptomatic and asymptomatic individuals, suggesting, therefore, that people who do not have symptoms may be able transmit the virus to others.”
OK, nothing new here. We’ve known asymptomatic people can transmit.
The recent briefing then states “asymptomatically-infected individuals are much less likely to transmit the virus than those who develop symptoms.” This is congruent with what Dr. Kerkhove states.
Ok, got it- It’s not impossible, but much less likely to transmit if you don’t have symptoms.
Let’s look a bit closer at the data they’re using so we can determine what “less likely” and “very rare” actually mean:
“Among 63 asymptomatically-infected individuals studied in China, there was evidence that 9 (14%) infected another person.”
“Furthermore, among two studies which carefully investigated secondary transmission from cases to contacts, one found NO secondary transmission among 91 contacts of 9 asymptomatic cases, while the other reported that 6.4% of cases were attributable to pre-symptomatic transmission.”
The WHO goes on to say, “The available data, to date, on onward infection from cases without symptoms comes from a limited number of studies with small samples that are subject to possible recall bias and for which fomite transmission cannot be ruled out.”
Conclusion: As of right now, we’re landing somewhere between 0% and 14% likelihood of transmitting Coronavirus if you’re asymptomatic, and perhaps about 6% if you’re pre-symptomatic. HOWEVER, they state we can’t rule out fomite transmission (touching a surface or object that was infected) AND we probably can’t trust the participant’s memory either (recall bias).
Well that’s a very long winded way for The WHO to say- we sort of think you might be less likely to transmit the virus (maybe) if you don’t have symptoms but there’s really not enough data so we probably shouldn’t say anything yet.
