Scientists working on a vaccine for the Zika virus say it could be available for emergency use before the end of the year.

It is being developed by US and Canadian researchers - who are hoping to begin human trials in September.

The World Health Organisation is predicting that up to four million people could be infected with Zika this year.

It's been linked with brain defects in babies.

But Jeremy Farrar from the medical research charity, the Wellcome Trust, believes it'll be difficult to achieve: