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The World Health Organization clarified comments an official made on Monday that called asymptomatic transmission of the coronavirus "very rare," saying in a press conference that these carriers do take part in spreading the virus but that more information is needed to know by how much.

What they're saying: WHO official Maria Van Kerkhove clarified Tuesday that patients sometimes confuse not having any symptoms with only exhibiting mild symptoms. In addition, some patients transmit the virus before developing symptoms. Contact tracers classify this group as "presymptomatic," rather than asymptomatic.

  • Van Kerkhov said the WHO estimates 16% of people are asymptomatic and can transmit the virus. Some models suggest up to 40% of coronavirus transmission might be due to asymptomatic spread, she added, but much more information is needed.
  • Van Kerkhove stressed that her comments on Monday were specific to particular studies and did not represent a new policy or direction. The WHO said it regrets saying that asymptomatic spread is "very rare."

Why it matters: The WHO's comments about asymptomatic transmission caused mass public confusion, as experts online pointed to several studies and modeling that have shown asymptomatic spread occurs.

  • Van Kerkhove did say Monday that preliminary evidence from the earliest outbreaks showed people who don't show symptoms aren't the "main driver" of new infections.
  • But the entire transcript from Monday shows she was stressing that governments should focus on detecting and isolating those with symptoms because asymptomatic people can be difficult to trace.

The big picture: After the press conference Monday, many in the public health community expressed disbelief that the WHO would disregard asymptomatic patients, especially since the threat of asymptomatic spread has shaped many government lockdown policies.

  • Several studies have shown that asymptomatic spread exists, including one from the Annals of Internal Medicine that showed a likelihood that approximately 40% to 45% of those infected with SARS-CoV-2 will remain asymptomatic and can spread the virus unknowingly.
  • Harvard's Global Health Institute wrote in a memo responding to the WHO's comments that "all of the best evidence suggests that people without symptoms can and do readily spread SARS-CoV2."
  • The director of the institute Ashish Jha wrote in a Twitter thread that asymptomatic spread is the "Achille's heal of this outbreak," and called on the WHO to provide data when making statements that could affect public behavior.

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Illustration: Sarah Grillo/Axios

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The scientific race for a coronavirus vaccine is moving at record-shattering speed. Making the most of that work — translating a successful clinical product into real-world progress — will require some patience.

Why it matters: If we get a vaccine relatively soon, the next big challenge will be balancing the need to get it into people's hands with the need to keep working on other solutions that might prove more effective.

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Data: The COVID Tracking Project, state health departments; Map: Andrew Witherspoon, Sara Wise/Axios

Coronavirus infections ticked up slightly over the past week, thanks to scattered outbreaks in every region of the country.

Where it stands: The U.S. has been making halting, uneven progress against the virus since August. Overall, we're moving in the right direction, but we're often taking two steps forward and one step back.