WHO clarifies comments about asymptomatic COVID-19 spread

June 10, 2020

World Health Organization (WHO) officials have clarified comments from a recent media briefing that portrayed asymptomatic spread of COVID-19 as rare, triggering concerns among epidemiologists and health providers about the accuracy of the information and unclear messaging for the public. Two WHO officials addressed questions about comments during a Facebook live session. The comments came after a reporter asked about asymptotic spread related to a media report from Singapore that said a number of asymptomatic cases were identified during recent large-scale testing.

During the briefing, Maria Van Kerkhove, PhD, the WHO's technical lead for COVID-19, said countries were finding asymptotic cases during contact tracing and that, based on unpublished data, secondary spread from people who have no symptoms appears to be very rare. However, the group has said transmission from asymptomatic and presymptomatic people does occur, a factor that underpins distancing recommendations and face mask advice. In the wake of the comments, several scientists and health providers questioned the WHO's interpretation and information sources and worried that the comments added confusion to crucial public health messaging, especially in light of mask recommendations.

During the Facebook live session, Van Kerkhove and Mike Ryan, MD, who directs the WHO's health emergencies program, clarified comments and answered questions. Van Kerkhove said that although most transmission involves sick people passing infectious droplets, there is an undetermined subset of people who do not ever have symptoms who pass the virus to others. She said the level of spread from asymptomatic people is a major unknown, and that what she didn't mention in the media briefing was that some modeling groups have tried to estimate the proportion, and there's a big range, with some groups estimating around 40 percent of transmission may be due to asymptomatic cases.

Another big question is what proportion of people are truly asymptomatic and never progress to having any symptoms and what proportion reported as asymptomatic actually have mild disease. "We're six months into a pandemic, there's a huge amount of research that's being done, but we don't have that full picture yet," Van Kerkhove said.

The second big question is what proportion of those who do not have symptoms actually transmit, she added. "And that's a big open question, and it remains an open question." What is known is that studies have found that patients shed virus one to three days before their symptoms begin, Van Kerhhove said, though a better understanding is still needed about what proportion of that is contributing to transmission.

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