After Criticism, WHO Official Issues Clarification on Asymptomatic Coronavirus Transmission

Maria Van Kerkhove, the WHO’s technical lead on the COVID-19 pandemic.  (Fabrice Coffrini/Getty)

A top World Health Organization official clarified on Tuesday that scientists have not determined yet how frequently people with asymptomatic cases of COVID-19 pass the disease on to others, a day after suggesting that such spread is “very rare.”

The clarification comes after the WHO’s original comments incited strong pushback from outside public health experts, who suggested the agency had erred, or at least miscommunicated, when it said people who didn’t show symptoms were unlikely to spread the virus.

Maria Van Kerkhove, the WHO’s technical lead on the COVID-19 pandemic, made it very clear Tuesday that the actual rates of asymptomatic transmission aren’t yet known.

“The majority of transmission that we know about is that people who have symptoms transmit the virus to other people through infectious droplets,” Van Kerkhove said. “But there are a subset of people who don’t develop symptoms, and to truly understand how many people don’t have symptoms, we don’t actually have that answer yet.”

Van Kerkhove’s remarks on Tuesday came at a WHO question-and-answer session aimed at explaining what was known and unknown about how the virus spreads.


Some of the confusion boiled down to the details of what an asymptomatic infection actually is, and the different ways the term is used. While some cases of COVID-19 are fully asymptomatic, sometimes the word is also used to describe people who haven’t started showing symptoms yet, when they are presymptomatic. Research has shown that people become infectious before they start feeling sick, during that presymptomatic period.

At one of the WHO’s thrice-weekly press briefings Monday, Van Kerkhove noted that when health officials review cases that are initially reported to be asymptomatic, “we find out that many have really mild disease.” There are some infected people who are “truly asymptomatic,” she said, but countries that are doing detailed contact tracing are “not finding secondary transmission onward” from those cases. “It’s very rare,” she said.

She added: “We are constantly looking at this data and we’re trying to get more information from countries to truly answer this question.”

To some, it came across as if the WHO was suggesting that people without symptoms weren’t driving the spread. Some studies, however, have estimated that people without symptoms (whether truly asymptomatic or presymptomatic) could be responsible for up to half of the spread, which is why the virus has been so difficult to contain. Isolating people who are sick, for example, does not prevent the possibility they already passed the virus on to others. Some modeling studies have assumed quite widespread asymptomatic transmission.

“The WHO created confusion yesterday when it reported that asymptomatic patients rarely spread the disease,” an email from the Harvard Global Health Institute said Tuesday. “All of the best evidence suggests that people without symptoms can and do readily spread SARS-CoV-2, the virus that causes COVID-19. In fact, some evidence suggests that people may be most infectious in the days before they become symptomatic — that is, in the presymptomatic phase when they feel well, have no symptoms, but may be shedding substantial amounts of virus.”

Their point: People not showing symptoms can spread the virus, whether they ultimately feel sick or not. That’s why wearing masks and keeping distance are so important to limiting transmission.

Van Kerkhove acknowledged Tuesday that her use of the phrase “very rare” had been a miscommunication. She said she had based that phrasing on findings from a small number of studies that followed asymptomatic cases and tracked how many of their contacts became infected. She said she did not mean to imply that “asymptomatic transmission globally” was happening rarely, because that has not been determined yet.

Determining which routes of transmission are driving most of the spread of the virus is crucial so health experts can develop the right strategies to combat the virus. At the WHO event Tuesday, officials stressed that even as scientists are still learning more about the virus and how it spreads, countries have demonstrated that the best tactics to address the pandemic include isolating cases, contact tracing, and quarantining contacts — as well as hand hygiene and respiratory etiquette.

With those measures, “We won’t stop all transmission, but what we do is, we suppress transmission,” Mike Ryan, the head of WHO’s emergencies program, said Tuesday.

Other unknowns the WHO experts raised Tuesday include how asymptomatic or presymptomatic people are spreading the virus if they are not coughing — it could be that they still expel infectious droplets through singing, yelling, or even talking — as well as the percentage of all COVID-19 cases that are asymptomatic. One recent paper estimated that 40% to 45% of cases might be asymptomatic, though others have pegged that figure at closer to 20% or even lower.

Based on what’s been seen so far, asymptomatic people tend to be younger and not have other health issues, Van Kerkhove said Tuesday, though she cautioned she didn’t want to generalize.

In addition to the studies about asymptomatic transmission that Van Kerkhove referred to, the WHO has also received data from member countries that “suggests that asymptomatically-infected individuals are much less likely to transmit the virus than those who develop symptoms,” according to WHO’s interim guidance on masks issued last week.

Outside experts have called on the WHO to release that data. Both outside experts and WHO officials have acknowledged that detecting asymptomatic spread would be really difficult, and just because scientists haven’t seen something occurring often doesn’t mean it’s not happening.

“Every question we answer, we have 10 more,” Van Kerkhove said Tuesday.

This story was originally published by STAT, an online publication of Boston Globe Media that covers health, medicine, and scientific discovery.