COVID-19. Photo: Image Point Fr - LPN/BSIP/Universal Images Group via Getty Images

Editor’s note: The WHO clarified the comments it made, as reported below, saying that asymptomatic carriers do take part in spreading the coronavirus. Read more here.

Contact tracing data from around the globe suggests that while there are instances of asymptomatic coronavirus patients transmitting the virus to others, they are not "a main driver" of new infections, World Health Organization officials said at a press conference Monday.

Why it matters: Evidence early on suggested that person-to-person transmission among people who didn't experience symptoms could lead to outbreaks that would be difficult to control. Young people and healthy people who did not experience symptoms were also suspected to be potential carriers to more vulnerable populations.

The big picture: The WHO is now relying on data obtained through contact tracing, said Maria Van Kerkhove, head of the emerging diseases and zoonosis unit.

  • “We have a number of reports from countries who are doing very detailed contact tracing. They’re following asymptomatic cases. They’re following contacts. And they’re not finding secondary transmission onward. It’s very rare."
  • In the press conference Monday, Kerkhove said there are fewer asymptomatic patients than previously thought.
  • "[I]f you actually go back and say how many of them are truly asymptomatic, we find out that many have really mild disease, very mild disease, they’re not quote unquote COVID symptoms, meaning they may not have developed fever yet."

Between the lines: Don't treat these statements as a permission to treat a lack of symptoms as a "get out of social distancing" free card.

  • Infected people can be contagious well before experiencing symptoms.
  • "Some modeling studies suggest 40-60% of spread is from people when they didn’t have symptoms," tweeted Ashish Jha, incoming dean at the Brown School of Public Health.
  • Singapore's coronavirus task force also said Monday that it believes half of the country's new COVID-19 cases are asymptomatic based on testing data, Reuters reports.

Van Kerkhove later tweeted a statement that cited a June summary by the WHO on symptomatic, pre-symptomatic and asymptomatic transmission:

"Comprehensive studies on transmission from asymptomatic individuals are difficult to conduct, but the available evidence from contact tracing reported by Member States suggests that asymptomatically-infected individuals are much less likely to transmit the virus than those who develop symptoms."

The bottom line: These statements are a reminder of just how little we understand about this virus.

Go deeper

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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.

Coronavirus cases increase in 17 states

Expand chart
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.

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Coronavirus dashboard

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  1. Politics: The swing states where the pandemic is raging — Pence no longer expected to attend Barrett confirmation vote after COVID exposure.
  2. Health: 13 states set single-day case records last week
  3. Business: Where stimulus is needed most.
  4. Education: The dangerous instability of school re-openings.
  5. States: Nearly two dozen Minnesota COVID cases traced to 3 Trump campaign events
  6. World: Restrictions grow across Europe.
  7. Media: Fox News president and several hosts advised to quarantine.

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