The hunt for a vaccine to prevent the Zika virus could take years, a top US health chief said Thursday amid a worrying outbreak of the mosquito-borne illness blamed for birth defects.
There is no vaccine or treatment for Zika, which the World Health Organization said is "spreading explosively" through the Americas and may lead to as many as four million cases in the region.
Zika can cause microcephaly - abnormally small heads and brains - in babies born to infected women.
Anthony Fauci, the director of the National Institute for Allergy and Infectious Diseases (NIAID), said the US government is working on two approaches toward a vaccine against Zika, based on research already done on related mosquito-borne viruses.
The first is a "DNA-based vaccine using a strategy very similar to what we employed for another flavivirus, the West Nile virus," he told reporters. Flaviviruses are generally transmitted by mosquitoes or ticks.
"Secondly, a live attenuated vaccine, building on similar and highly immunogenic approaches used for the closely related dengue virus," he added.
Hopes are high that a so-called Phase I clinical trial could begin later this year to test the safety and efficacy of a Zika vaccine in people, but Fauci cautioned that a finished product will take far longer.
"While these approaches are promising, it is important to understand that we will not have a widely available, safe and effective Zika vaccine this year and probably not even in the next few years," he said.
The US Centers for Disease Control and Prevention (CDC) has warned women who are pregnant or want to become pregnant to avoid traveling to areas in Latin America and the Caribbean that are experiencing outbreaks of the virus.
Although the Zika virus was first documented in 1947, it caused only sporadic and small outbreaks of illness until recently. Little is known about it.
"Please take this seriously," said CDC principal deputy director Anne Schuchat.
"It is very important that you understand that we don't know as much as we want to know about this yet, and while we are learning more it is prudent to consider postponing travel."
Some airlines are offering refunds to expectant mothers with tickets to any of 22 countries or territories with outbreaks of the disease.
Brazil experienced its first outbreak of Zika last year and has seen the number of microcephaly cases soar, from 163 a year on average to more than 3,718 suspected cases, according to the health ministry.
A total of 31 cases of Zika have been documented in the United States since last year, all involving people who were infected while they were out of the country, Schuchat said.
In the future, "it is possible, even likely, that we will see limited Zika outbreaks in the US," Schuchat said, particularly in the southern parts of Florida and Texas.
Fauci said the United States typically spends $97 million per year on viruses that are transmitted by mosquitoes and ticks, and it will draw from that pool of money to fund new Zika research in a host of areas, including diagnostic tests, vaccines, basic research and vector control.
Zika virus ‘spreading explosively’: 4m under threat
The Zika virus is ‘spreading explosively’ in the Americas, the head of the World Health Organisation said on Thursday, as the global health body warned that it expected up to four million cases of the disease.
WHO chief Margaret Chan called for an emergency meeting on February 1 on the outbreak of the virus, which has been blamed for the birth defect microcephaly, a condition that causes babies to be born with an abnormally small head.
"The level of alarm is extremely high," Chan said, adding that the meeting of WHO's Emergency Committee on Monday will seek to determine if the outbreak qualifies as an international public health emergency.
The virus ‘is now spreading explosively,’ in the Americas, where 23 countries and territories have reported cases, the WHO chief said.
Marcos Espinal, head of communicable diseases and health analysis at WHO's Americas office, said the region should expect ‘three to four million cases’ of Zika, without proving a timeframe for the outbreak to ramp up to that level.
Situation 'dramatically different'
Following its initial discovery in a monkey in Uganda's Zika forest in 1947, the disease ‘slumbered’ and ‘occasionally caused a mild disease of low concern,’ in humans, Chan said.
"The situation today is dramatically different," she said.
Chan highlighted the growing concern over Zika's possible link to microcephaly and a neurological disorder called Guillain-Barre syndrome.
"A causal relationship between Zika virus infection and birth malformations and neurological syndromes has not yet been established, but is strongly suspected," Chan said.
The emergency meeting will seek advice on the severity of the outbreak and what response measures might be taken.
It will also aim to identify priority areas for urgent research, Chan said, after US President Barack Obama called for swift action, including better diagnostic tests as well as the development of vaccines and treatments.
Espinal warned that Zika "will go everywhere the mosquito is."
"We should assume that. We should not wait for it to spread," he said.
Drawing a contrast with Ebola, Espinal stressed that Zika needs a carrier to spread and that controlling the mosquito was therefore crucial to controlling the outbreak.
WHO has previously said that it expects Zika to spread to every country in the Americas except Canada and Chile.
Brazil has been the country hardest hit so far, and concerns are growing about this summer's Olympics, which is likely to attract hundreds of thousands of visitors to host city Rio de Janeiro in August.
Zika originated in Africa and also exists in Asia and the Pacific, but has not been associated with microcephaly there. It first came to prominence in Brazil in October.
Microcephaly can cause brain damage or death in babies.
In Brazil, cases of microcephaly have surged from 163 a year on average to more than 3,718 suspected cases since the outbreak, according to the health ministry.
Colombia, Ecuador, El Salvador and Jamaica have warned women to avoid getting pregnant for the time being.