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. The virus is primarily spread via respiratory droplets when someone coughs or sneezes or if they touch a contaminated surface, scientists say. WHO officials say Covid-19 can also spread in the so-called pre-symptomatic stage - a few days before a patient shows symptoms. 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."