“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, Van Kerkhove said, and tracking anyone who might have come into contact with them. Van Kerkhove acknowledged that there are some studies that have indicated asymptomatic or presymptomatic spread in nursing homes and in household settings.