An official leading the World Health Organization’s COVID-19 response clarified comments made at a media briefing Monday that suggested coronavirus transmission by asymptomatic individuals is “very rare.”
Dr. Maria Van Kerkhove, WHO’s technical lead, said in a video posted on the organization’s Twitter feed that she was referring to a small subset of studies and unpublished information from member states when making the comments.
“I wasn’t stating a policy of WHO or anything like that, I was just trying to articulate what we know,” she said. “And in that I used the phrase ‘very rare,’ and I think that’s a misunderstanding to state that asymptomatic transmission globally is very rare.”
In contact tracing efforts from other countries, secondary transmission from asymptomatic individuals was rarely found, Kerkhove said, but the subject is still “a major unknown.”
She said models that attempt to estimate asymptomatic transmission vary greatly – some predict spread by people without symptoms may make up to 40% of transmissions.
Monday’s comments fueled tweets by anti-lockdown activists questioning the country’s decision to shutter businesses in the name of public health, leading to record unemployment and job loss.
“We shut ournation down, nearly destroyed the economy, and have people wearing masks for no reasons other than govt. control,” tweeted Newsmax TV host John Cardillo.
Some public health experts noted Kerkhove’s failure to support her claim with published studies, while others rushed to her defense.
“It makes sense,” Dr. Faheem Younus, chief of infectious diseases at the University of Maryland Upper Chesapeake Health, said on Twitter. “Each cough may emit ~3000 and sneeze ~40,000 droplets from a symptomatic patient. The risk is much less with asymptomatic.”
Kerkhove reiterated some of her points in Tuesday’s clarification video, including that some pre-symptomatic cases may have been misidentified as asymptomatic.
Other individuals, she said, may have been misidentified as asymptomatic because they didn’t present common COVID-19 symptoms such as fever, cough and shortness of breath, instead exhibitingmuch milder symptoms.
Kerkhove said much research is being done to determine when someone has COVID-19 and is truly asymptomatic and what proportion of that population actually transmits the disease.
“That’s a big open question, and that remains an open question,” she said.
Many experts on Twitter criticized Kerkhove for not differentiating between asymptomatic and pre-symptomatic cases as multiple studies suggest people who don’t exhibit any symptoms are able to transmit the disease.
A report published in the New England Journal of Medicine in late May found more than half of residents with positive coronavirus test results at a nursing home in Washington were pre-symptomatic at the time of testing and most likely contributed to transmission.
A small study published in the peer-reviewed journal Nature in April found COVID-19 patients may be most contagious two to three days before symptoms appear. Researchers estimated 44% of transmissions in their study occurred during the index patient’s pre-symptomatic stage.
Additionally, the Centers for Disease Control and Prevention estimates in planning scenarios that 40% of virus transmissions occur prior to symptom onset.
“That’s why it’s important to avoid people as much as possible, to wash your hands, to wear masks,” Tara Smith, a professor of epidemiology at Kent State University. “To prevent you from spreading the virus.”