Transcript, Dr. Van Kerkhove on asymptomatic transmission

Transcript, Dr. Maria Van Kerkhove's claims regarding asymptomatic transmission of COVID-19 while answering questions as part of a World Health Organization press conference on 2020-06-08 (Video: https://www.youtube.com/watch?v=NQTBlbx1Xjs):

So there's a couple of things in the question that you just asked. One is the number of cases that are reported that are being reported as asymptomatic. And so, we hear from a number of countries that X number, X percentage of them, are reported as not having symptoms, or that they are in their pre-symptomatic phase, which means it's a few days before they actually develop severe symptoms.

In a number of countries, when we go back and we discuss with them, one, how are these asymptomatic cases being identified, many of them are being identified through contact tracing — and so — which is what we would want to see, in that you have a known case, you find your contacts, they're already in quarantine, hopefully, and some of them are tested, and then you pick up people who may have asymptomatic, or no symptoms, or even mild symptoms.

The other thing we're finding is that when we actually go back and say, how many of them were truly asymptomatic we find out that many have really mild disease — very mild disease — they're not, quote unquote "Covid symptoms", meaning they may not have developed fever yet, they may not have had a significant cough or they may not have shortness of breath. But some may have mild disease. Having said that we do know that there can be people that are truly asymptomatic and PCR-positive.

The second part of your question is, what proportion of asymptomatic individuals actually transmit. So the way that we look at that is we look at — they need, these individuals need to be followed carefully, over the course of when they're detected and looking at secondary transmission.

We have a number of reports from countries who are doing very detailed contact tracing. They're following asymptomatic cases, they're following contacts, and they're not finding secondary transmission onward — it's very rare. And that, not — much of that is not published in the literature. From the papers that are published, there's one that came out from Singapore, looking at a long-term care facility, there's some household transmission studies, where you follow individuals over time, and you look at the proportion of those that transmit onwards.

We are constantly looking at this data and we're trying to get more information from countries to truly answer this question. It still appears to be rare that an asymptomatic individual actually transmits onward. What we really want to be focused on is following the symptomatic cases.

If we followed all of the symptomatic cases — because we know that this is a respiratory pathogen, it passes from an individual through infectious droplets — if we actually followed all of the symptomatic cases, isolated those cases, follow the contacts and quarantine those contacts, we would drastically reduce — I would love to be able to give a proportion of how much transmission we would actually stop. But it would be a drastic reduction in transmission. If we could focus on that, I think we would do very, very well in terms of suppressing transmission. But from the data we have it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual.