"Perhaps in three years we will have a vaccine," Jorge Kalil, head of the Butantan Institute in Sao Paulo, told reporters in Geneva, acknowledging that even that estimate was "optimistic".
He was among global health experts meeting in Geneva since Monday to determine what research and development should be prioritised in the fight against Zika, which has been spreading most rapidly in the Americas.
Zika was previously only known to cause moderate cold and flu-like symptoms, but increasing evidence indicates the virus may be connected to multiple neurological disorders, as well as microcephaly, a severe birth defect in which babies are born with smaller heads and brains.
WHO's deputy director for health systems and innovation Marie-Paule Kieny told reporters work was under way to develop an emergency vaccine "product profile" to help stake out regulatory requirements and help guide developers.
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She said the final profile should be ready in May, but acknowledged that developing a usable vaccine would take much longer.
"Vaccine developement is still at an early stage and the most advanced candidates are still months away from entering early human clinical trials," Kieny said.
WHO said today that 18 companies and research institutes were currently working on Zika vaccines. None have been tested on humans.
Another 31 labs are working on developing diagnostic tests, the global health body said, with a profile on the needed diagnostic tools expected to be ready by mid-April.