(Reuters) - The first vaccine against dengue fever, from France's Sanofi, provided moderate protection in a large clinical study, but questions remain as to how well it can help fight the world's fastest-growing tropical disease.

The late-stage trial involved 10,275 healthy children aged 2-14 across five countries in Asia, a region that accounts for over two-thirds of the mosquito-borne disease's global burden.

Sanofi had already disclosed in April that its vaccine reduced the incidence of dengue fever by 56 percent in the Asian study, without giving details. The full findings were published online on Friday in The Lancet medical journal.

The study found the vaccine was safe and reduced the most serious cases of haemorrhagic fever by nearly 90 percent. But it offered poor protection to young children - who are most at risk from dengue - and proved notably inefficient in tackling one of the viral disease's four strains.

The results suggest the new vaccine acts best as an immune booster for patients with some previous exposure, and therefore may be most useful in tropical regions where dengue is common, rather than as a vaccination for travellers.

Since there is no vaccine now, that could still make it an important weapon in the fight against dengue.

"In view of the high disease burden in endemic countries... this vaccine candidate, despite moderate overall efficacy, could have a substantial effect on public health," the scientists behind the study wrote.

The study was led by Maria Rosario Capeding from the Research Institute for Tropical Medicine in the Philippines, and funded by Sanofi.

Nearly half the world's population is at risk of contracting dengue fever - also known as "breakbone fever" because of the severe pain it can cause. The disease infects some 100 million people each year, according to the World Health Organization, and some experts put the number at triple that level.

Most patients survive dengue but it kills an estimated 20,000 people each year, many of them children, and causes one hospitalization every minute around the globe.

Sanofi has invested over 1.3 billion euros ($1.77 billion) in the past 20 years in developing the vaccine, which is several years ahead of potential competitors, and has built a dedicated factory near Lyon in southern France with capacity to produce 100 million doses a year.

The company plans to establish a separate division within its vaccines business to manage the rollout of the new product.

Some analysts believe the three-dose vaccine could sell 1 billion euros a year, significantly boosting Sanofi's vaccines business, which generated sales of 3.7 billion euros in 2013.

But there are many uncertainties about commercial prospects - including whether three shots of what is likely to be a pricey vaccine are really needed, since one of the findings from the latest study was that efficacy after at least one dose was almost as high as after three doses.

Sanofi is still testing its vaccine in 20,000 children in Latin America and Guillaume Leroy, head of its dengue vaccine program, said it would await the results of that final study - due by the year-end - before seeking regulatory approval in endemic countries next year.

ENIGMA

Coming up with a vaccine that can protect against the disease has puzzled scientists for over 70 years, since the movement of troops during World War II helped dengue spread across the Pacific and become a worldwide pandemic.

In the Asian study, the vaccine provided only 35 percent protection against serotype 2.

A 2012 study in Thailand had already shown that the vaccine was unable to tame serotype 2, which happened to be most prevalent at the time of the study. This time, serotype 2 was less prevalent among patients, drawing the vaccine's overall efficacy rate higher.

Annelies Wilder-Smith of Singapore's Nanyang Technological University, who wrote a commentary accompanying the Lancet study, said its failure in serotype 2 was "an enigma".

Questions remain as to what the threshold of dengue incidence might be for countries to decide that it is worth launching costly vaccination programs, she noted.

Difficulties in developing a dengue vaccine echo similar problems that GlaxoSmithKline has had in developing the first vaccine for malaria, which also did not perform as strongly in clinical trials as initially hoped.

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(Additional reporting by Ben Hirschler in London, editing by David Evans)