Medical Examiner

The World Health Organization Didn’t Actually Mean Asymptomatic Spread Is “Very Rare”

Here’s what went wrong at that press conference.

A protective face mask thrown in the street.
You still need that mask! Damien Meyer/Getty Images

On Monday, the World Health Organization held a press conference suggesting that asymptomatic transmission is “very rare,” according to infectious disease expert Maria Van Kerkhove. It seemed like great news. Then, whiplash: On Tuesday, WHO walked that back. On the question of how often asymptomatic transmission occurs, “we don’t actually have that answer yet,” said Van Kerkhove.

If you are understandably confused, here’s the bottom line: People without clear symptoms spread the virus, and they do it often. This is why we’re all wearing masks, and why we should keep doing that, in addition to taking other precautionary measures. But you also might be wondering: Why did WHO seemingly reverse course without seeing new data? What exactly went wrong? The answer is: a few things! It (mostly) boils down to an issue of how terms are used.

The first is how you define asymptomatic. In the initial press conference, Van Kerkhove drew a distinction between people who are “truly asymptomatic” and people who “have really mild disease … they’re not quote unquote COVID symptoms,” like severe cough and shortness of breath. She didn’t clarify what symptoms those people would have instead, but it seems like her definition excludes people who are, on close inspection, a little sick.

Then there’s the confusion of the categories asymptomatic and presymptomatic. The first, technically speaking, describes people who never show symptoms of the virus, while the second describes people who just haven’t shown symptoms yet. The trouble is that in common use asymptomatic also can refer to people who simply don’t currently have symptoms (and obviously, it’s difficult to know which will be which). The lede of a Monday CNBC story summed up WHO’s initial declaration as “coronavirus patients without symptoms aren’t driving the spread of the virus.” The rest of the article also fails to clearly delineate between asymptomatic spread and presymptomatic spread, even at one point explicitly lumping the two terms together. This confusing article went viral, with an Ohio representative tweeting the link along with “reopen America!”

But how the two groups spread or don’t spread the virus might be different—we simply don’t know for sure. “There is ample evidence that presymptomatic people commonly infect others,” wrote Eric Topol in a thread on Twitter parsing the difference between the two terms. He pointed to a study of 94 patients with moderate cases of COVID-19. In cases where they passed the coronavirus onto others, about half the time it happened before they had symptoms, the researchers estimated.

We know less about how asymptomatic people spread the disease. When Van Kerkhove said such transmission is “very rare,” she was apparently referring to “a small subset of studies” that look at data from contact tracing, as CNBC clarified in a follow-up story. She was not referring to models that estimates how much asymptomatic spread could be occurring. This distinction matters because contact tracing and data on asymptomatic patients is indeed lacking even though some models estimate that asymptomatic carriers may be responsible for 40 percent of transmission. (As of this writing, that story clarifying that asymptomatic patients spread the virus sits below the initial CNBC story about asymptomatic patients rarely spreading the virus in the site’s “trending now” list.)

She could have spelled this out much more clearly, yes. But another problem here is that her original remarks were quickly taken out of context. She was not telling people to rush out to the bars because they don’t have to worry about asymptomatic spread. She was making a point about where she thought large-scale efforts to find and isolate people should be focused. In any case, what she was describing “seems more of scientific than practical interest,” biostatistician Natalie Dean noted on Twitter. On an individual level, we shouldn’t change our behavior.