A sharp increase in birth defects in Brazil has triggered a global health emergency over the mosquito-borne virus and spurred a race to develop a vaccine and better diagnostic tests.
"We have a few more weeks to be sure to demonstrate causality, but the link between Zika and Guillain-Barre is highly probable," Marie-Paule Kieny, WHO Assistant Director-General for Health Systems and Innovation, told a news briefing.
Kieny said U.S. government scientists and an Indian biotechnology company were currently frontrunners in vaccine development, although it would take at least 18 months to start large-scale clinical trials of candidate shots.
"Two vaccine candidates seem to be more advanced: a DNA vaccine from the U.S. National Institutes for Health and an inactivated product from Bharat Biotech in India," she said.
The NIH is working on a DNA-based vaccine that uses the same approach as one being developed for West Nile virus. India's Bharat said last week that its experimental vaccine would start pre-clinical trials in animals imminently.
Overall, around 15 groups are working on Zika vaccines, including France's Sanofi (SASY.PA), as well as researchers in Brazil, who announced a new partnership with the University of Texas on Thursday.