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WHO official: Asymptomatic spread of coronavirus 'very rare'

A top World Health Organization (WHO) official on Monday said that it appears “very rare” for an asymptomatic person with the coronavirus to transmit it to another person, a potential bit of good news in the fight against the virus.

“From the data we have, it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual,” said Maria Van Kerkhove, the WHO’s technical lead for COVID-19, when asked about the issue at a press briefing.  

She noted that the answer is not definitive. “We are constantly looking at this data, and we're trying to get more information from countries to truly answer this question,” she said. “It still appears to be rare that an asymptomatic individual actually transmits onward.”

She said the WHO has information reported by countries that has not been published in studies, finding that detailed contact tracing has not found significant spread from asymptomatic people. 

“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,” Van Kerkhove said. “It's very rare. Much of that is not published in the literature.”

However, Ashish Jha, director of the Harvard Global Health Institute, expressed some skepticism of the WHO’s claim and said he thinks asymptomatic transmission is, in fact, an important source of spread and that some modeling shows as much as 40 to 60 percent of transmission is from people without symptoms. 

Jha said it’s possible the WHO is making a distinction between asymptomatic spread and presymptomatic spread, when someone eventually develops symptoms but spreads the virus before they do. 

If it is indeed true that asymptomatic spread of the virus is very rare, it would make it easier to contain the transmission because there would be less worry about people unwittingly spreading the virus as they went about their lives without any symptoms. 

“If this turns out to be true it would be a game-changer, but I think it would be really important for us to know whether CDC concurs?” tweeted Peter Hotez, an infectious disease expert at Baylor University, in response to the WHO statement on Monday. 

Van Kerkhove said the focus should be on tracking the symptomatic cases. 

“If we actually followed all of the symptomatic cases, isolated those cases, followed the contacts and quarantined those contacts, we would drastically reduce [transmission],” she said.