The WHO Is Sending Mixed Messages About Asymptomatic Spread—Here Are the Facts

On April 3, the CDC issued its official recommendation that all people start wearing non-medical masks in public to help slow the spread of the novel coronavirus, because, it said, asymptomatic spread—transmission of COVID-19 by people who were infected but would never show any symptoms—was a big contributor to new cases. The agency cited seven recent studies on outbreaks around the world in its announcement, all of which suggest significant transmission by asymptomatic and pre-symptomatic patients. Ever since, most people in the United States have worn cloth masks over their mouths and noses to not only avoid inhaling the respiratory droplets that spread the virus, but to keep from unknowingly passing the virus on themselves.

On Monday, a World Health Organization official issued a statement that asymptomatic spread was actually “very rare,” which felt like the first major piece of good news during the pandemic. After more than four months of social distancing (in most states) and two months of wearing cloth masks in increasingly hot weather, it felt like we were finally catching a break.

The news was particularly welcome in the wake of weeks of police brutality protests, which public health officials supported even as they were concerned about the spread of the virus in crowds. The asymptomatic transmission statement seemed to suggest that as long as people who were physically sick stayed home, the risk of catching the virus was a lot lower. That turned out to be not quite right.

The WHO’s original statement failed to acknowledge a crucial difference between totally asymptomatic carriers and those who are simply in the early stages of their infection and haven’t yet experienced their first symptom. After swift pushback from public health experts, the WHO walked back its assertion that asymptomatic spread is nothing to worry about, clarifying that scientists haven’t yet determined how common it is.

The WHO now says that asymptomatic spread is still a significant concern, despite the announcement on Monday—and, given the outcry of public health experts outside of the WHO, we’re all better off ignoring the organization’s strange optimism about the rarity of asymptomatic spread. Here’s a simple breakdown of the two key arguments against the WHO’s statement and subsequent waffling this week, based on what is known about COVID-19 at this point in time.

Asymptomatic does not equal pre-symptomatic

The first crucial point is the difference between carriers who are truly asymptomatic—as in, people who have confirmed cases of coronavirus but did never, and would never, experience any symptoms at all—and those who are in the pre-symptomatic stage of the illness, who will experience symptoms in the near future, but are in the limbo stage between catching the virus and actually getting sick.

Based on virus case studies, researchers believe the incubation period, or the time between exposure and the first symptom, is a wide-ranging three to 14 days. According to a study published in the Annals of Internal Medicine in early March, the average incubation period is around five days. This finding helped shape understanding of how many infected people could be slipping through temperature checks as they travel through airports.

Many subsequent studies show that pre-symptomatic patients absolutely spread the virus: One widely cited study, published in Nature Medicine in April, suggested that people are most contagious about two days before the onset of symptoms, and that 44 percent of cases are perhaps spread in this window.

Asymptomatic doesn’t mean “mildly symptomatic”

Even as hundreds of thousands of people become so severely ill from COVID-19 that they die, both the CDC and WHO maintain that most infections are mild and require no medical intervention. Some cases are so mild—especially among young, healthy people—that the illness can be mistaken for something harmless, like simple fatigue or allergies. In a Twitter thread about the WHO announcement, Andy Slavitt, the former administrator of the Centers for Medicare and Medicaid Services, said the organization admits it is unable to distinguish between asymptomatic and low-symptom patients. Regular people are typically bad at properly reporting their own symptoms. The only way to know if someone is truly asymptomatic is via clinical monitoring and testing.

As the New York Times points out, the WHO has been slow to adapt its rules to evolving science during the pandemic. “On the one hand, I do want to cut the WHO some slack, because it is hard to do this in an evolving pandemic,” Ashish Jha, director of the Harvard Global Health Institute, told the Times. “At the same time, we do rely on the WHO to give us the best scientific data and evidence.”

It may—with more study and time spent with this virus—turn out that asymptomatic spread is not as grave a concern as scientists believed months ago, at the start of the pandemic. But, once again: This virus is extremely new. As we've seen throughout this pandemic, information changes rapidly, and it's often better to be safe in the ways that are confirmed to help than to disregard precautions based on shaky-feeling updates that contradict earlier guidance from one week to the next. Staying home, wearing a mask, and maintaining social distancing measures remain the best ways to avoid spreading the virus and getting sick yourself.

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