This story has been updated to include a statement from the WHO and clarifying statements from Dr. Maria Van Kerkhove.
A World Health Organization leader walked back statements she made Monday that implied that it was "rare" for asymptomatic coronavirus patients to transmit the disease to others.
"I used the phrase 'very rare.' I think that's misunderstanding to state that asymptomatic transmission globally is very rare. What I was referring to was a subset of studies," Dr. Maria Van Kerkhove, a WHO expert on infectious diseases, said during a press conference on Tuesday. "I was just trying to answer a question at a press conference, I wasn't stating a policy of the WHO or anything like that."
Van Kerkhove statement comes a day after she caused a stir in the medical community by implying that asymptomatic was not a common way the virus was spread.
— World Health Organization (WHO) (@WHO) June 9, 2020
"We have a number of reports from countries who are doing very detailed contact tracing," Van Kerkhove said Monday. "They're following asymptomatic cases. They're following contacts. And they're not finding secondary transmission onward. It's very rare."
On Monday, Dr. Ashish Jha, the director of Harvard's Global Health Institute, took issue with Van Kerkhove's anaylsis
"Asymptomatic spread is the Achille's heel of this outbreak," Jha said. "Both asymptomatic and pre-symptomatic spread huge problem for controlling disease because folks shedding virus while asymptomatic pre-symptomatic has one advantage: You can use contact tracing to find folks they infected. But that doesn't help prevent pre-symptomatic spread"
Jha cited a May Swiss study that indicated that between 40 and 60% of all coronavirus cases originated from people when they didn't have symptoms at the time of the spread.
Van Kerkhove said that based on the data she has seen, in many cases, when they go back and find coronavirus cases through contact tracing, many infected with the virus have "really mild disease."
"They are not 'COVID' symptoms," Van Kerkhove said. "They may not have developed a fever yet, they may not have had a significant cough or they may not have shortness of breath. But some may have mild disease."
Van Kerkhove said that the WHO is still requesting countries to contribute data to allow the organization to better understand the spread of the virus.
"What we really want to be focused on is following the symptomatic cases," she said. "If we followed all the symptomatic cases, because we know it is a respiratory pathogen, it passes to a person from infectious droplets. If we actually followed all of the symptomatic cases, isolated those cases, followed the contacts and quarantined those contacts, we would drastically reduce" the outbreak.
In a statement released early Tuesday morning, the WHO said that "current evidence suggests that most transmission of COVID-19 occurs from symptomatic people to others in close contact, when not wearing appropriate PPE."
It added that while it is possible for asymptomatic people to spread the virus, "infection from cases without symptoms comes from a limited number of studies with small samples that are subject to possible recall bias," and in most cases where asymptomatic spread was found, it couldn't be ruled out that the person contracted the virus through other ways.
Last month, the CDC released data that indicated that 35% of those infected with the virus are asymptomatic.