Asymptomatic spread of coronavirus is 'very rare,' WHO says

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.

Some people, particularly young and otherwise healthy individuals, who are infected by the coronavirus never develop symptoms or only develop mild symptoms. Others might not develop symptoms until days after they were actually infected.

Preliminary evidence from the earliest outbreaks indicated that the virus could spread from person-to-person contact, even if the carrier didn't have 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."

Government responses should focus on detecting and isolating infected people with symptoms, and tracking anyone who might have come into contact with them, Van Kerkhove said. She acknowledged that some studies have indicated asymptomatic or presymptomatic 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."

If asymptomatic spread proves to not be a main driver of coronavirus transmission, the policy implications could be tremendous. A report from the U.S. Centers for Disease Control and Prevention published on April 1 cited the "potential for presymptomatic transmission" as a reason for the importance of social distancing.

"These findings also suggest that to control the pandemic, it might not be enough for only persons with symptoms to limit their contact with others because persons without symptoms might transmit infection," the CDC study said.

To be sure, asymptomatic and presymptomatic 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.