An expert with the World Health Organization on Monday made brief comments about asymptomatic spread of COVID-19 that sparked a firestorm of confusion, backlash, and criticism.
Some public health experts were quick to lash out at the organization for poor messaging. Others tried to clarify what the WHO expert might have been trying to say. And still others quickly impeached evidence-based strategies to combat the pandemic virus.
On Tuesday, the WHO responded with a live Q&A on social media to address confusion and lingering questions about transmission. In it, the WHO expert who made the confusing comments on Monday tried to clarify the issue and add context and caveats. But the response may still leave some confused and frustrated.
Here, we’ll try to clear the air around what was said, what maybe should have been said, and what we do and don’t know about transmission of the new coronavirus, SARS-CoV-2.
What we know
First and foremost: people who are infected with SARS-CoV-2 but do not have symptoms can—and do—spread the virus to other people. To put it another way: people who seem healthy and do not have classic COVID-19 symptoms may actually be infected and can still pass on the virus to other people, infecting them.
We know this is possible, and we know that it happens. No public health expert is saying otherwise—including those at the WHO.
Moreover, we know that there are two scenarios in which this can happen.
Some people who are infected with SARS-CoV-2 will never develop symptoms from their infection—that is, from the time they are infected with the virus to the time that the virus is no longer reproducing in their cells and, thus, they are no longer infected. The WHO considers these cases “asymptomatic.” If asymptomatic people spread the virus to someone else during their silent infection, that spread would be considered “asymptomatic transmission.”
In the other scenario, people who are infected and initially do not have symptoms later go on to develop symptoms, sometimes very mild ones. In fact, data suggests that a person can test positive for the infection one to three days before symptoms develop. Symptoms tend to develop between five to six days after an exposure to the virus, but the process can take as long as 14 days. Studies have found that viral shedding—how much infectious virus particles are spewing out of an infected person—appears to be highest in the few days around the first day of symptoms.
That means that infected people can test positive for the virus and spread the virus to others before their symptoms develop.
So far, so good
Here’s where this gets confusing.
Regardless of when in the course of an infection an infected person develops symptoms, they are considered to have a symptomatic case overall.
But if they test positive while they have no symptoms and then go on to develop symptoms, they would be considered “pre-symptomatic” at the time of the testing. If there’s no follow-up with the case to know if they ever develop symptoms, they could be misclassified as “asymptomatic.”
If they spread the virus to someone else while they are “pre-symptomatic,” that could be considered “pre-symptomatic transmission” or “asymptomatic transmission” because they were asymptomatic at the time of transmission.
I’m using “could” in these sentences because different experts and studies use these terms differently. See how confusing this is?
Then, of course, there are the straight symptomatic cases that we understand very well. From these cases, it’s clear that SARS-CoV-2 spreads in large respiratory droplets, launched out of a mouth or nose from sneezing, coughing, loud talking, or heavy breathing. We know that the these droplets tend to not go farther than one to two meters away from an infected person. So, the most effective way to prevent this type of transmission is to keep physical distance from others who may be infected. When physical distancing is not possible, experts recommend a face mask.
So, to sum up:
Symptomatic case = Someone who is infected and has symptoms at some point.
Asymptomatic case = Someone who is infected but never develops symptoms.
Pre-symptomatic = The phase of a symptomatic infection when a person may test positive for the virus and/or may spread the virus but has not yet developed symptoms.
Pre-symptomatic transmission = Spread of the virus from a symptomatic case during their pre-symptomatic phase.
Asymptomatic transmission = Spread of the virus from an infected person with no current symptoms. This transmission could be from a pre-symptomatic person or a truly asymptomatic case, depending on how the terms are being used.
The WHO has consistently used asymptomatic transmission only when talking about truly asymptomatic cases.
What we don’t know
There’s a lot about transmission we don’t know.
For one thing, we don’t have a clear picture of how many infected people have symptomatic cases and how many have asymptomatic cases. Estimates of the percentage of cases that are asymptomatic vary wildly, with some ranging from 4 percent to 45 percent.
From there, we’re not certain which types of cases are transmitting the infection and when—that is, we don’t know what proportion of asymptomatic cases spread the infection to others. Some data suggest a small fraction—such as 6.4 percent—of asymptomatic cases transmit the virus, while other modeling data estimates that 40 percent of all transmission in the pandemic is from asymptomatic cases.
For symptomatic cases, we don’t know what proportion is spreading the infection before they develop symptoms. This, too, has a large range of estimates.
The WHO has held that, according to its view of the data, symptomatic cases still appear to be behind the majority of new infections, whether that occurs during the pre-symptomatic phase or not. As such, the organization pushes the proven strategies of isolating symptomatic cases and tracing, quarantining, and testing contacts.
What the WHO initially said
Amid all this uncertainty and confusion, the issue of asymptomatic transmission came up in a regular WHO press conference on Monday, June 8.
A reporter with Reuters noted that a Singapore health official had reported that nearly half of new cases there appeared to be asymptomatic. The reporter asked the WHO “whether it’s possible that [asymptomatic cases] have a bigger role than the WHO initially thought in propagating the pandemic and what the policy implications of that might be?”
Kerkhove—who is an epidemiologist—first tried to define the case terms above—asymptomatic, pre-symptomatic—and explain that the WHO doesn’t take the use of the term “asymptomatic” at face value. But her response was muddled and fragmented.
“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, which is what we want to see, that you have a known case, you find your contacts, they’re already in quarantine hopefully and some of them are tested,” she said. “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 go back and say, how many of them were truly asymptomatic, we find out that many have really mild disease, they’re not—quote, unquote—COVID symptoms, meaning they may not have developed ever yet, they may not have had a significant cough or they may not have shortness of breath but some may have mild disease.”