In a news briefing on Monday, a W.H.O. official asserted that transmission of the coronavirus by people without symptoms is “very rare.” Following concerted pushback from researchers, officials on Tuesday walked back the claim, saying it was a “misunderstanding.”
But it is not the first time the W.H.O.’s assessment has seemed to lag behind scientific opinion.
The agency delayed endorsing masks for the general public until Friday, claiming there was too little evidence that they prevented transmission of the virus. Virtually all scientists and governments have been recommending masks for months.
The W.H.O. has said repeatedly that small airborne droplets, or aerosols, are not a significant factor in the pandemic’s spread, although a growing body of evidence suggests that they may be.
“The W.H.O. has been out of step with most of the world on the issue of droplets and aerosols,” said Michael Osterholm, an infectious disease expert at the University of Minnesota.
These scientific disagreements have wide policy implications. Many countries, including the United States, adopted lockdown strategies because they recognized that isolating only people who were sick might not be enough to contain the epidemic.
If the virus is transmitted by small airborne droplets, people will need to continue to avoid congregating in poorly ventilated spaces, even if they practice rigorous hand hygiene.
The W.H.O. traditionally has taken a cautious approach to evaluating scientific evidence. But the pace of research has changed: Now scientists are rushing to publish preliminary research, even before their results can be thoroughly vetted by other experts.
The avalanche of findings may bring advances — like a vaccine — in record time. But the onslaught also has led to confusion, even retractions of high-profile results.
“On the one hand, I do want to cut the W.H.O. some slack, because it is hard to do this in an evolving pandemic,” said Dr. Ashish Jha, director of the Harvard Global Health Institute. “At the same time, we do rely on the W.H.O. to give us the best scientific data and evidence.”
The W.H.O.’s thinking on asymptomatic transmission does not appear to have changed much since February, when the W.H.O. China Joint Mission reported that “the proportion of truly asymptomatic infections is unclear, but appears to be relatively rare and does not appear to be a major driver of transmission.”
Studies later estimated this number could be as high as 40 percent; the current best estimate from the Centers for Disease Control and Prevention is 35 percent. The research prompted many countries, including the United States, to endorse use of masks by everyone.
But on Monday, Dr. Maria Van Kerkhove, the W.H.O.’s technical lead for coronavirus response, said that “it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual.”
Her statement provoked an immediate backlash from scientists, who noted that study after study had shown transmission of the virus from people before they ever felt symptoms.
The reaction prompted the W.H.O. to clarify its position in a live session hosted on Facebook and Twitter. Dr. Van Kerkhove said her comment had been based on only two or three studies.
“I was just responding to a question, I wasn’t stating a policy of W.H.O. or anything like that,” she said. Dr. Van Kerkhove said her statement was also based on unpublished evidence that some countries have shared with the W.H.O.
But critics, including its own officials, said the organization should be transparent about its sources. “W.H.O.’s first and foremost responsibility is to be the science leader,” said Lawrence Gostin, director of the W.H.O. Collaborating Center on National and Global Health Law.
“And when they come out with things that are clearly contradicted by the scientific establishment without any justification or citing studies, it significantly reduces their credibility.”
A key point of confusion is the difference between people who are “pre-symptomatic” and will go on to develop symptoms, and those who are “asymptomatic” and never feel sick. Dr. Van Kerkhove suggested that her comments were about people who are truly asymptomatic.
A widely cited paper published in April suggested that people are most infectious up to two days before the onset of symptoms, and estimated that 44 percent of new infections were a result of transmission from people who were not yet showing symptoms.
W.H.O. refers to such people as pre-symptomatic. “OK, technically fine,” Dr. Jha said. “But for all intents and purposes, they are asymptomatic — they are without symptoms.”
Dr. Van Kerkhove said that by using the two terms, W.H.O. officials are in fact trying to be very clear about the group of people they are referring to.
“Unfortunately, that’s not how everybody uses it,” she said. “I didn’t intend that to make things more complicated.”
The W.H.O. continues to maintain that large respiratory droplets expelled by sneezing or coughing are the main route of transmission and to downplay a possible role for aerosols, smaller particles that may linger in the air.
But evidence is piling up that aerosols may be an important route.
“What they haven’t recognized is that activities like coughing and talking, even breathing in some cases, are also aerosol-producing procedures,” said Linsey Marr, who studies airborne transmission of viruses at Virginia Tech.
W.H.O. officials said they were aware that breathing and talking might result in aerosols, but questioned their importance in spreading the virus.
“To date, there has been no demonstration of transmission by this type of aerosol route,” said Dr. Benedetta Allegranzi, the W.H.O.’s technical lead on the coronavirus.
But the W.H.O. defines airborne transmission too narrowly, some scientists said. Airborne transmission also includes the possibility that the virus is aloft for shorter distances, then inhaled.
“They have a very early 20th century, very unsophisticated view of what aerosols and airborne transmission are,” said Dr. Don Milton, an expert on public health aerobiology at the University of Maryland.
Up until the 1950s, Dr. Milton said, tuberculosis was thought to be spread by prolonged close contact. “We now know that it’s only transmitted by aerosols,” he said.
Some scientists are suspicious that W.H.O.’s stance on masks and aerosols may stem less from scientific research than from a concern over supplies of personal protective equipment for medical workers.
The organization currently recommends respirator masks that would block aerosols only for health care workers doing medical procedures that produce aerosols.
Dr. Van Kerkhove said that the W.H.O.’s guidance was based only on science and not on any considerations of supply. While a shortage of P.P.E. is a problem, she said, “it doesn’t change what we recommend.”
All of the experts said it was not that the W.H.O. is wrong on all counts, but that given the implications of its statements, it should be more cautious in concluding that transmission by air or by people without symptoms is not significant.
“We don’t know,” Dr. Milton said. “But they also don’t know.”
Some experts said that when the W.H.O. uses the phrase “there is no evidence” to indicate uncertainty, it is in fact conveying certainty about the absence of a phenomenon.
Dr. Van Kerkhove conceded that point.
“That’s a fair statement,” she said. “There’s a lot of research that needs to be done to really understand this, and we are open to the fact that there is new research every day.”