You may have seen headlines recently that “patients without symptoms” aren’t driving the spread of the coronavirus. That would seem to suggest that all our measures about masks and distancing are useless—but that’s a misunderstanding of the evidence
The World Health Organization’s technical lead on COVID-19, Maria Van Kerkhove, answered a question at a press conference on Friday in which she said that “asymptomatic” spread is “very rare.” Today, in a dedicated Q&A session, she said it was a mistake to use those exact words, and clarified what data they were based on. You can watch the whole thing here:
The WHO’s understanding of transmission, including how much of it may be asymptomatic, is discussed on page 2 of their new mask guidance. Unfortunately, it is not very clear about how much transmission comes from people without symptoms, and the document acknowledges that there are only a few available studies on the matter. Those studies, meanwhile, have enough limitations that we can’t say for sure how many people are truly asymptomatic or how likely those people are to spread the virus.
What we actually know
People who get the virus and show symptoms seem to be infectious for one to three days before their symptoms appear. They are then infectious while they are sick and possibly for some time afterward. One study found that people with “mild” symptoms were infectious for eight days after the onset of symptoms, with the highest viral load toward the beginning of their illness. Sicker people may be infectious for longer.
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To break this down, there are three groups of people who can spread COVID-19 to others:
People who are currently sick (symptomatic) People who have been infected with the virus, but will never show symptoms (asymptomatic)People who have been infected with the virus and will develop symptoms, but they don’t know it yet (pre-symptomatic)The first group–symptomatic patientsz–is responsible for “most” of the spread, the WHO says. Specifically: people with symptoms, who are in close contact with others and who aren’t using personal protective equipment like masks.
But the other groups still contribute. We just don’t yet know exactly how much.
There have only been a few studies looking at truly asymptomatic people. Based on these and some unpublished data, the WHO says that “asymptomatically-infected individuals are much less likely to transmit the virus than those who develop symptoms.” But it’s impossible to pin down a specific number.
Pre-symptomatic people are another story, though. It’s possible that everybody who gets sick will spend a bit of time in that infectious, pre-symptomatic state. They could be walking around assuming they’re healthy, when in fact they’re at their highest viral load and will start to develop a fever or other symptoms within a few hours or days.
That, the WHO’s Mike Ryan said today, is why COVID-19 has spread so widely around the globe. For other recent diseases of concern, like Ebola and SARS, you don’t tend to spread them until you are actually experiencing symptoms. Transmission from people who don’t currently have symptoms is therefore an important part of the picture, no matter the exact numbers.
To recap: according to the WHO’s interpretation, most infections come from people who are currently sick. Some come from people who aren’t sick yet, or who will never feel sick.
What this means for masks and other recommended measures
All the measures you’ve heard about are still important. If you are sick, you can spread the virus, so it’s important to stay home if possible. If you do not currently feel sick, you should still observe distancing, wear a mask and follow all of the other recommendations. It’s definitely possible for people who feel fine to spread the virus, whether they’re asymptomatic or pre-symptomatic.
You can also be sick—with symptoms—and not realize you have COVID-19. Some people with a mild case of the virus may just feel a bit under the weather or think their allergies are acting up more than usual. You may feel good enough to go and do some shopping, but in a study you’d be counted among the symptomatic patients.
So while the WHO’s announcement may sound confusing, nothing has really changed. Continue to be especially careful if you or a close contact are sick, and continue to follow guidelines—including those about masks and physical distancing—even if you feel well.
