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.

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