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COVID-19: What do we know about asymptomatic transmission?

The W.H.O has walked back a remark made Monday about asymptomatic transmission being "very rare."

MINNEAPOLIS — The World Health Organization official who sparked widespread confusion Monday by calling asymptomatic transmission of the novel coronavirus "very rare" has clarified that remark.

"We do know that some people who are asymptomatic, or don't have symptoms, can transmit the virus," said Dr. Maria Van Kerkhove during a briefing Tuesday. Van Kerkhove said her remark Monday, in response to a question during that day's briefing, referred to just "two or three studies."

She said while there are many unknowns yet about how the virus is transmitted, she cited some models during the briefing which she says estimate as much as 40% of transmission comes from asymptomatic spread.

"We had both a statement which was incomplete and headlines which took it and ran with it, when in fact nothing has changed in terms of our understanding as far as how this virus can spread," said Dr. Josh Michaud, Associate Director for Global Health Policy with the Kaiser Family Foundation. "We know that people who don't have symptoms ... can transmit this virus to some extent. We don't know yet to what extent that is, but it does happen."

A study from CDC scientists found evidence supports transmission by people not showing symptoms. 

But there is a difference between asymptomatic and presymptomatic. People who are asymptomatic have the virus, but never show symptoms. People who are presymptomatic have the virus, but aren't showing symptoms yet

"It might be easier to think of this as people who are unaware that they're infected, either because they don't recognize the symptoms, because [the symptoms are] very mild, or because they truly are asymptomatic," said Dr. Tim Schacker, Vice Dean for Research at the University of Minnesota Medical School. "That almost certainly accounts for a significant portion of new infections."

A study published in April estimated 44% of COVID-19 infections came from people who were presymptomatic and "inferred that infectiousness peaked on or before symptom onset."

That's why, despite all the unknowns, public health officials recommend wearing masks in public, avoiding crowded places, and practicing social distancing. 

"The public health message at its core hasn't changed," said Michaud.

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