• Government responses should focus on detecting and isolating infected people with symptoms, the World Health Organization said.
  • Preliminary evidence from the earliest outbreaks indicated the virus could spread even if people didn't have symptoms.
  • But the WHO says that while asymptomatic spread can occur, it is "very rare."

World Health Organization officials on Tuesday walked back the comments below that were made on Monday after drawing criticism from epidemiologists across the world. Dr. Maria Van Kerkhove, head of the WHO's emerging diseases and zoonosis unit, said Tuesday that asymptomatic spread is a "really complex question" and much is still unknown. "We don't actually have that answer yet," she said. 

"I was responding to a question at the press conference. I wasn't stating a policy of WHO or anything like that. I was just trying to articulate what we know," she said on a live Q&A streamed across multiple social media platforms. "And in that, I used the phrase 'very rare,' and I think that that's misunderstanding to state that asymptomatic transmission globally is very rare. I was referring to a small subset of studies."

Here is our followup article Tuesday and below is the original story published Monday.

Read more: WHO walks back comments on asymptomatic coronavirus spread, says much is still unknown

Coronavirus patients without symptoms aren't driving the spread of the virus, World Health Organization officials said Monday, casting doubt on concerns by some researchers that the disease could be difficult to contain due to asymptomatic infections. 

Preliminary evidence from the earliest outbreaks indicated that the virus could spread from person-to-person contact, even if the carrier never develops symptoms. But WHO officials now say that while asymptomatic spread can occur, it is not the main way it's being transmitted. 

"From the data we have, it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual," Dr. Maria Van Kerkhove, head of WHO's emerging diseases and zoonosis unit, said at a news briefing from the United Nations agency's Geneva headquarters. "It's very rare."

The virus is primarily spread via respiratory droplets when someone coughs or sneezes or if they touch a contaminated surface, scientists say. 

Asymptomatic transmission is particularly worrisome for public health officials, leading many to institute severe lockdowns and policies requiring masks in public. That's because those patients never develop symptoms and, in many cases, don't even know they are infected. WHO officials say Covid-19 can also spread in the so-called pre-symptomatic stage — a few days before a patient shows symptoms. 

Van Kerkhove said government officials should still focus on detecting and isolating infected people with symptoms, and tracking anyone who might have come into contact with them. The WHO also revised its guidance on masks last week, saying they should be worn in public places, especially on public transportation and in densely populated areas. She acknowledged that some studies have indicated that there's been asymptomatic or pre-symptomatic spread in nursing homes and in household settings.

More research and data are needed to "truly answer" the question of whether the coronavirus can spread widely through asymptomatic carriers, Van Kerkhove added.

"We have a number of reports from countries who are doing very detailed contact tracing," she said. "They're following asymptomatic cases. They're following contacts. And they're not finding secondary transmission onward. It's very rare."

Research from the U.S. Centers for Disease Control and Prevention contradicts Kerkhove's statement. An April 1 report from the CDC cited the "potential for presymptomatic transmission" as a reason for the importance of social distancing. The CDC cited research showing asymptomatic transmission among family members and in nursing homes and other facilities as a key factor in recommending face coverings, widespread testing and other safety measures, according to an article published May 4

"If the COVID-19 pandemic is found to be driven by undetected asymptomatic or mildly symptomatic SARS-CoV-2 infections, new innovations in disease detection and prevention (beyond exhaustive contact tracing, mass testing, and isolation of asymptomatic contacts) may be needed," the CDC said in its May 4 article.

If asymptomatic spread, however, proves not to be a big driver of coronavirus transmission, the policy implications could be tremendous.

To be sure, asymptomatic and pre-symptomatic spread of the virus appears to still be happening, Van Kerkhove said but remains rare. That finding has important implications for how to screen for the virus and limit its spread. 

"What we really want to be focused on is following the symptomatic cases," Van Kerkhove said. "If we actually followed all of the symptomatic cases, isolated those cases, followed the contacts and quarantined those contacts, we would drastically reduce" the outbreak.

Correction: This article was updated to include a more complete explanation of asymptomatic and pre-symptomatic transmission as well as to change the headline. An earlier version of the headline should have said most asymptomatic coronavirus patients aren't spreading new infections. The word "most" was inadvertently omitted.