Zika outbreak declared a global emergency

The World Health Organisation has declared the spread of the Zika virus a global health emergency, calling for a co-ordinated international response to halt its progress.

The virus, which is transmitted by the Aedes mosquito, has dominated headlines in the last few weeks, and has been linked to a surge in cases of microcephaly in South America. The disorder is characterised by an abnormally small head in newborns and can lead to lifelong developmental issues.

There have been around 4,000 reported cases of microcephaly in Brazil alone since October, and now The World Health Organisation (WHO) has said that it is highly likely – but not yet scientifically proven - that the cases are caused by pregnant women being infected by the virus.

Despite this uncertainty, WHO's Director General Margaret Chan has acted swiftly in declaring the outbreak an emergency. The agency was heavily criticised for the slowness of its response to the Ebola outbreak in West Africa in 2014, where experts say faster action would have saved lives.

Most Zika infections are mild and cause few or no symptoms, however there have also been reported cases of Guillain-Barre syndrome, a rare neurological disorder which causes paralysis.

Speaking after an emergency meeting, Chan announced: "After a review of the evidence, the Committee advised that the recent cluster of microcephaly cases and other neurological disorders reported in Brazil, following a similar cluster in French Polynesia in 2014, constitutes an "extraordinary event" and a public health threat to other parts of the world."

Outbreaks of the virus have already been reported in more than 20 countries across South America, Central America and the Southern portion of the US.

WHO has now issued extensive guidelines for action and advice, but says the most important protective measures are to control mosquito populations and the prevention of mosquito bites in at-risk individuals, especially pregnant women.

It has not called on any general travel restrictions to halt the disease's spread, however pregnant women have been advised to consider delaying travel to affected areas.

How soon can a vaccine be developed?

Since the first reports of the virus hit the headlines, pharmaceutical companies working in the field have been bombarded with queries about how long a vaccine against Zika would take to develop.

However, unlike Ebola, few vaccines companies have candidates which can be developed quickly, with the big players such as GSK and Sanofi saying an effective vaccine could be years away.

Small US company Inovio is an exception this. It says it has a vaccines already in development, and predicts that it could be ready by the end of 2016.

WHO has also called for the development of a diagnostic test to confirm cases of the disease quickly, something that would help efforts to track and contain the disease.

The upcoming El Nino weather patterns seen in South America are adding to the concerns over the outbreak potential of the Zika virus. Populations of the Aedes mosquito – which also causes the dengue fever - are expected to increase dramatically due to the increase in temperature.