*Text courtesy Reuters

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(Reuters) - The world's first vaccine against dengue fever, being developed by French drugmaker Sanofi SA, protected against three of the virus's four strains in a keenly awaited clinical trial in Thailand.

Sanofi said on Wednesday the proof of efficacy was a key milestone in the 70-year quest to develop a viable dengue shot, adding the results also confirmed the safety profile of its vaccine candidate, which could reach the market in 2015.

Other drug companies are also working on dengue vaccines but Sanofi's product is several years ahead.

The mosquito-borne disease, also known as "breakbone fever", is a threat to nearly 3 billion people and is caused by four different types of virus, none of which confers immunity from the others.

Sanofi's vaccine generated an antibody response for all four dengue virus types, but evidence of protection was only demonstrated against three of the four strains circulating in Thailand.

Duane Gubler of the Duke-N.U.S. Graduate Medical School, who has researched dengue for four decades, said the results looked "great", despite the failure to defend against all strains.

"I am not at all concerned about the lack of protection against all four serotypes. Based on what we know about the immune response to dengue viruses, if the vaccine protects against three serotypes, recipients will be protected against severe disease," he said in an emailed exchange.

Sanofi said researchers were carrying out analyses to understand the lack of protection for the fourth serotype.

"It's a surprise," company spokesman Pascal Barollier said. "We need to get to the bottom of the data to find out why it is reacting this way and wait for ongoing Phase III trials to see if it is linked to some specific situation in Thailand."

The Phase IIb study involving 4,002 Thai children aged four to 11 years was conducted during a dengue epidemic, which might be an explanation for the unexpected outcome.

Deutsche Bank analyst Mark Clark said the lack of protection against the fourth virus type meant a commercial launch was more likely in 2015 than in 2014, as Sanofi awaits Phase III data rather than filing early in some countries.

"More positively, given that protection against at least three of the four viral types has been demonstrated, the data supports the likelihood of launch for this huge unmet clinical need," Clark wrote in a research note.

TROPICAL MEGA-CITIES

The company's vaccine unit Sanofi Pasteur has already invested 350 million euros ($423 million) in a new French factory to make the three-dose vaccine and believes its product could generate more than 1 billion euros in yearly sales.

But uptake of the vaccine will depend on precisely how well doctors believe it can protect populations at risk in fast-expanding tropical cities from Rio to Manila, as well as travelers to such areas.

Sanofi, which reports second-quarter results on Thursday, gave no details on the level of protection in a brief statement. The full data are now being reviewed by scientific experts and public health officials, with detailed results to be published later this year.

Barollier said the aim was to publish the study results in a scientific journal in September and then present the research to the American Society of Tropical Medicine and Hygiene in Atlanta in November.

Large-scale late-stage Phase III clinical studies with 31,000 participants are under way with Sanofi's vaccine in 10 countries in Asia and Latin America.

Dengue fever, which can cause intense joint and muscle pain, is spread by the bite of the Aedes aegypti mosquito. The insect thrives in the mega-cities of the tropics, with the result that nearly half the world's population are at risk of catching the disease.

In the past 50 years there has been a 30-fold jump in dengue cases. The World Health Organization officially puts infections at between 50 and 100 million a year, though many experts think this assessment from the 1990s badly under-estimates the disease.

Most patients survive but it is estimated to kill about 20,000 every year, many of them children less able to fight it off.

(Editing by David Holmes/Catherine Evans)