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”, - said Dr. Maria Van Kerkhove, head of WHO’s emerging diseases and zoonosis unit. - “It’s very rare”.