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* Mosquito-borne virus linked to birth defects in Brazil
* Brazil's Butantan Institute aims to develop vaccine
* GSK and Sanofi reviewing vaccine possibilities
* More research needed into potential sexual transmission (Adds comment from GlaxoSmithKline and Sanofi, graphic link)
By Tom Miles and Ben Hirschler
GENEVA/LONDON, Jan 25 (Reuters) - The mosquito-borne Zikavirus, which has been linked to brain damage in thousands ofbabies in Brazil, is likely to spread to all countries in theAmericas except for Canada and Chile, the World HealthOrganization said on Monday.
Zika transmission has not yet been reported in thecontinental United States, although a woman who fell ill withthe virus in Brazil later gave birth to a brain-damaged baby inHawaii.
Brazil's Health Ministry said in November that Zika waslinked to a foetal deformation known as microcephaly, in whichinfants are born with smaller-than-usual brains.
Brazil has reported 3,893 suspected cases of microcephaly,the WHO said last Friday, over 30 times more than in any yearsince 2010 and equivalent to 1-2 percent of all newborns in thestate of Pernambuco, one of the worst-hit areas.
The Zika outbreak comes hard on the heels of the Ebolaepidemic in West Africa, demonstrating once again howlittle-understood diseases can rapidly emerge as global threats.
"We've got no drugs and we've got no vaccines. It's a caseof deja vu because that's exactly what we were saying withEbola," said Trudie Lang, a professor of global health at theUniversity of Oxford. "It's really important to develop avaccine as quickly as possible."
Large drugmakers' investment in tropical disease vaccineswith uncertain commercial prospects has so far been patchy,prompting health experts to call for a new system of incentivesfollowing the Ebola experience.
"We need to have some kind of a plan that makes (companies)feel there is a sustainable solution and not just a one-shotdeal over and over again," Francis Collins, director of the U.S.National Institutes of Health, said last week.
The Sao Paulo-based Butantan Institute is currently leadingthe research charge on Zika and said last week it planned todevelop a vaccine "in record time", although its director warnedthis was still likely to take three to five years.
British drugmaker GlaxoSmithKline said on Monday itwas studying the feasibility of using its vaccine technology onZika, while France's Sanofi said it was reviewingpossibilities.
RIO CONCERNS
The virus was first found in a monkey in the Zika forestnear Lake Victoria, Uganda, in 1947, and has historicallyoccurred in parts of Africa, Southeast Asia and the PacificIslands. But there is little scientific data on it and it isunclear why it might be causing microcephaly in Brazil.
Laura Rodrigues of the London School of Hygiene and TropicalMedicine said it was possible the disease could be evolving.
If the epidemic was still going on in August, when Brazil isdue to host the Olympic Games in Rio de Janeiro, then pregnantwomen should either stay away or be obsessive about covering upagainst mosquito bites, she said.
The WHO advised pregnant women planning to travel to areaswhere Zika is circulating to consult a healthcare providerbefore travelling and on return.
The clinical symptoms of Zika are usually mild and oftensimilar to dengue, a fever which is transmitted by the sameAedes aegypti mosquito, leading to fears that Zika will spreadinto all parts of the world where dengue is commonplace.
More than one-third of the world's population lives in areasat risk of dengue infection, in a band stretching throughAfrica, India, Southeast Asia and Latin America.
Zika's rapid spread, to 21 countries and territories in theAmericas since May 2015, is due to the prevalence of Aedesaegypti and a lack of immunity among the population, the WHOsaid in a statement.
RISK TO GIRLS
Like rubella, which also causes mild symptoms but can leadto birth defects, health experts believe a vaccine is needed toprotect girls before they reach child-bearing age.
Evidence about other transmission routes, apart frommosquito bites, is limited.
"Zika has been isolated in human semen, and one case ofpossible person-to-person sexual transmission has beendescribed. However, more evidence is needed to confirm whethersexual contact is a means of Zika transmission," the WHO said.
While a causal link between Zika and microcephaly has notyet been definitively proven, WHO Director-General Margaret Chansaid the circumstantial evidence was "suggestive and extremelyworrisome".
In addition to finding a vaccine and potential drugs tofight Zika, some scientists are also planning to take the fightto the mosquitoes that carry the disease.
Oxitec, the UK subsidiary of U.S. synthetic biology companyIntrexon, hopes to deploy a self-limiting geneticallymodified strain of insects to compete with normal Aedes aegypti.
Oxitec says its proprietary OX513A mosquito succeeded inreducing wild larvae of the Aedes mosquito by 82 percent in anarea of Brazil where 25 million of the transgenic insects werereleased between April and November. Authorities reported a bigdrop in dengue cases in the area.
(Editing by Mark Trevelyan)
