Post #711: “Asymptomatic cases” and the spread of COVID-19

If you read this blog, you realize I’ve been tracking the pandemic pretty closely.  So when the WHO was quoted today as saying that “asymptomatic cases don’t routinely spread COVID-19”, I said, why is that news?  They’ve been saying that for months.

 Then I read the coverage, and I realize that it’s news only because people don’t know what “asymptomatic cases” means.  And can’t be bothered to stop for ten minutes and figure it out.

Thus confirming the #1 problem in America today.  Everybody feels entitled to a firmly held and vehemently expressed opinion.  Nobody feels compelled to do even the tiniest bit of homework first.

So, let me fill in the homework.  As briefly as possible, “asymptomatic cases” means individuals who never, ever show any symptoms.  That does NOT mean “pre-symptomatic” cases, that is, individuals who are in the period between infection and onset of symptoms.  Guaranteed, 99% of what you read in the popular press will have confused the two.


In a nutshell

There’s a spate of news articles today based on some offhand remarks at a WHO press conference held this morning.  Or, more properly, based on a simple misunderstanding of what “asymptomatic cases” means.  So you’re seeing a lot of misinformation by people who think they know what that means, but who have not bothered to take ten minutes to track down what it actually means.

So, I interrupt epidemiology week to bring you this public service announcement.  When the WHO says “asymptomatic case” of COVID-19, they mean individuals who never, ever show any symptoms of the disease.  Full stop. 

Today, when the WHO said that those “asymptomatic cases” were not responsible for the spread of disease, that was not news.  The WHO has been saying that for months.  Colloquially, a person who doesn’t get sick from a disease, but spreads it, is a “Typhoid Mary”.  I documented the initial WHO finding of no “Typhoid Marys” on 3/16/2020 (Post #551, WHO report).  That’s how long this has been known.  That’s how much today’s off-the-cuff remarks by WHO staff are not news.  That’s how many months now that the WHO has been saying that “asymptomatic cases” — people who are infected but never show any symptoms — are not carriers of the disease.

To be clear, the WHO was NOT talking about the pre-symptomatic portion of an infection — the period of time between infection and onset of symptoms.  They are NOT talking about individuals who don’t show symptoms at the moment, but then go on to become ill from their COVID-19 infection.

The only reason this WHO press conference is news is that the popular press and blog-o-sphere have screwed up this exact point.  They have confused “asymptomatic cases” — people who never know they were infected, because they never have any symptoms — with “pre-symptomatic” cases, that is, persons who are infected, but aren’t yet showing symptoms.

There is abundant, clear evidence that pre-symptomatic cases are infectious.  Again, to be clear, a person can be walking around, with no apparent symptoms, and be spreading the disease.  E.g., here’s the US CDC talking about the importance of pre-symptomatic transmission of disease in Singapore, where it was specifically studied.

This confusion about what “asymptomatic cases” means has led some to conclude that if a person looks healthy, with no symptoms of COVID-19, then they can’t be spreading disease.  This is simply wrong.  See prior paragraph.

So, what did the WHO actually say? My paraphrase:  What they said was that a) people who never show symptoms (asymptomatic cases) almost never spread disease, and so b) to track the spread of infection via contact tracing, it is adequate to find the people who (eventually) show symptoms, and work backward from them.  I.e., you don’t have to worry about this potential pool of hard-to-find, hard-to-trace  “asymptomatic cases”, because they just don’t matter in terms of spreading the disease.

Detail follows.  I’m going to walk through this one, step by step, starting with the question at the WHO press conference.


More detail, with citations as to sources

WHO press conference.   The question about asymptomatic cases arises about 32 minutes into this WHO press conference, seen here via Twitter.  Briefly, the question is something like this:

“Singapore … now reports that half of their COVID-19 cases have no symptoms … wondering if this has a bigger role than the WHO initially thought.”

Peel that back another layer, just to be sure:  What did Singapore just report?  This morning, they reported that half of their new cases never showed any symptoms.  That was news-worthy because, initially at least, such asymptomatic cases were thought to be rare.

And so, again, to be crystal clear, the question at the WHO news conference, that the WHO was addressing, was about individuals who NEVER show symptoms, period.  And the question was, in effect, you used to think those were rare, but now that Singapore says they are not rare, do you think they might matter in the spread of disease?

And the WHO gave the answer that they have always given, and that appears to be backed up by the research:  No, these people don’t seem to be spreading the disease.  There are no “Typhoid Marys”, that is, individuals who never get sick, yet spread the disease.  I documented this portion of the WHO report on the Wuhan epidemic, on 3/16/2020 (Post #551, WHO report).

But this has nothing to do with the pre-symptomatic portion of an infection.  People are quite capable of spreading disease in the pre-symptomatic portion of an infection.  That is, those who will go on to develop symptoms can (and so) spread disease before those symptoms are apparent.

E.g., relevant to Singapore, here’s the US CDC talking about the importance of pre-symptomatic transmission of disease there.  In other words, it is well established that individuals can spread the disease before they develop symptoms.

What fraction is spread by pre-symptomatic individuals?  Tough to say.  Here’s an estimate of 68%, but it’s based on the experience in India. Earlier estimates had put that at somewhat less than half.  Ah, here’s a study for the fraction of cases in a nursing home due to pre-symptomatic transmission of disease.  Or maybe you’d prefer a recent review of the scholarly literature on this topic, from CDC.

So here’s the point.  Based on a misinterpretation of “asymptomatic case” to be “not currently showing symptoms”, you now have a large segment of the news media and blog-o-sphere trying to convince people that individuals who aren’t showing symptoms can’t be spreading disease.  And that’s not only wrong, it’s grossly wrong.  Hence this public service announcement.