A vaccine for the Zika virus is at least 18 months away from large-scale trials, according to the World Health Organization.

Marie-Paule Kieny, the WHO assistant director-general for health systems and innovation, said that 15 companies had been identified as having possible vaccines, during a press conference in Geneva on 12 February.

Two of the candidates "seem to be more advanced," she said, but ultimately still remain 18 months away from large scale trials.

"In spite of this encouraging landscape, vaccines are at least 18 months away from large-scale trials," said Kieny.

She also said it is becoming "more and more probable" that the Zika virus is linked to birth defects. It will take "a few more weeks" to officially understand if there is a causal link between the virus and microcephaly and Guillain-Barré syndrome.

The Zika virus is transmitted by the Aedes mosquito, and has spread across South and Central America rapidly since May 2015. On 1 February, the WHO responded by announcing an international health emergency.

Cases have been confirmed in Brazil, Costa Rica, Colombia, Denmark, the US, Ireland, the UK, Spain, Venezuela and China.

The Centres for Disease Control and Prevention (CDC) declared on 2 February that the Zika virus had been passed through sexual contract - the first recorded incident of its kind.