The United States has two potential candidates for a vaccine for the Zika virus and may begin clinical trials in people by the end of this year, but there will not be a widely available vaccine for several years, U.S. officials have admitted .

Dr. Anthony Fauci, director of the National Institute for Allergy and Infectious Disease, said one of the vaccines was based on work done on the West Nile virus.

Fauci said that vaccine was never developed because a drug company partner could not be found, but he did not see this as an issue for Zika.

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An infectious disease expert told Daily Mail Online that a vaccine for the fast-moving Zika virus - which has been linked to birth defects - will not be developed for close to a decade. That's because, unlike other viruses that have had recent outbreaks, Zika was assumed to be 'mild' and no vaccine had been in development

'We're already talking to a few companies who are able to partner with us in advanced development,' he told a news conference.

Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention, said there have been 31 cases of Zika infection among U.S. citizens who traveled to areas affected by the virus. So far, there have been no cases of transmission of the virus through mosquitoes in the United States, she said.

In Geneva, the World Health Organization (WHO) said on Thursday that Zika is spreading 'explosively' and could affect as many as four million people in the Americas.

It said the mosquito-borne disease had gone from a mild threat to one of alarming proportions and admitted up to four million people could be infected.

The agency will convene an emergency committee of disease experts on Monday.

It comes after US experts claimed WHO was not taking a leadership role in the Zika pandemic.

They said the organisation needed to learn lessons from its handling of the Ebola epidemic where the 'agency's failure to act decisively cost thousands of lives'.

The WHO says it has set up measures to fight the spread including surveillance, laboratories for testing, clinical care and vector control - killing off the mosquitoes spreading the infection.

But Dr Amesh Adalja, senior associate at the University of Pittsburgh Medical Center, told dailymail.com that officials shouldn't expect any fast results from vaccines.

Before the current outbreak – which is linked to a surge in babies being born with abnormally small heads – Zika was not considered to be a public health risk.

Dr Adalja told Daily Mail Online: 'There wasn't any vaccine development being undertaken prior to this outbreak in Brazil.

'It will be quite a while – and this is on the scale of about a decade for a vaccine to be developed usually.'

There is currently no vaccine or treatment for Zika, which is a close cousin of dengue and chikungunya, two viruses that cause mild fever and rash.

Zika virus is common in parts of Africa and South East Asia, but since 2007 there have been various outbreaks outside of the disease's comfort zone. It spread to South America in 2014, before reaching Mexico and the Caribbean last year. The first US case was reported in Texas at the start of January

Approximately 80 per cent of people who are infected with Zika have no symptoms.

The other 20 per cent experience 'mild' symptoms, such as fever, rash, joint pain, and red eyes.

As a result, there weren't any Zika vaccines in the pipeline, according to Dr Adalja.

But in recent months, the virus has been linked to a rare birth defect called microcephaly – in which a newborn's head is smaller than normal and the brain may not have developed properly.

The virus has been found in 24 countries in Latin America and the Caribbean.

The World Health Organization has warned it will soon spread to all countries across the Americas, except for Canada and Chile.

A lot of people might be thinking that all of the sudden they had an Ebola vaccine. This is different from Ebola – that vaccine had been in development for many years Dr Amesh Adalja, senior associate at the University of Pittsburgh Medical Center

Dr Adalja explained that the public incorrectly assumes a vaccine will be available shortly – because of the rapid vaccine that was made available shortly after the Ebola crisis struck.

He said: 'A lot of people might be thinking that all of the sudden they had an Ebola vaccine.

'This is different from Ebola – that vaccine had been in development for many years.'

But now that Zika has become a public health concern, researchers are going into overdrive.

First off, scientists will have to do 'some basic science' to see if parts of the virus can be presented to the immune system, according to Dr Adalja.

That phase of the research may involve animal studies in the laboratory – and will involve investigating different ways that the immune system may react to the virus.

And once the first step is completed, scientists will next have to determine whether the vaccine is safe or if it has a high burden of side effects.

They will also have to look into if the vaccine triggers any autoimmune reactions.

The virus is spread through the Aedes mosquito (pictured). As a vaccine won't be available anytime soon, the medical community should focus on wiping out the mosquito that transmits the disease, the expert said

Dr Adalja said: 'It takes time, and there are a lot of steps where the process can fail, but it seems to me that because of Zika's ability to affect fetal abnormalities, it will become a priority.'

He noted that the vaccine won't work just to limit the disease in adults – but also to limit the effect it can have on fetuses.

However, for the time being, there is no cure for Zika.

The doctor said: 'Once a person is infected, there is nothing that you can do.

It takes time, and there are a lot of steps where the process can fail, but it seems to me that because of Zika's ability to affect fetal abnormalities, it will become a priority Dr Adalja

'If a pregnant woman is infected, there is no treatment that can be done to protect the fetus – it either happens or it doesn't.'

The best way to prevent a fetus from getting infected right now is for pregnant women to avoid travelling to regions that are affected by the virus.

Dr Adalja added: 'While we wait for a vaccine, people really need to focus the battle on the mosquitoes that are transmitting the virus.'

A developer recently revealed it created a genetically modified mosquito that reduces the number of mosquitoes spreading Zika.

Oxitec, a UK-subsidiary of US synthetic biology company Intrexon, said it developed a self-limiting strain of the Aedes aegypti mosquito.

The male mosquitoes were modified so that their offspring will die before reaching adulthood or being able to reproduce.

The company said it saw strong results in controlling the population of the Aedes vector that carries Zika and also the dengue virus.