* Dengue world's fastest-growing tropical disease
* Infects some 100 mln people a year, according to WHO
By Natalie Huet
PARIS, Sept 3 (Reuters) - French drugmaker Sanofi, developing the first vaccine against dengue fever, said its product reduced disease cases by 60.8 percent in a large final clinical trial.
Sanofi has invested more than 1.3 billion euros ($1.7 billion) in the project, undertaking two decades of research on the world's fastest-growing tropical disease.
The final study - conducted on 20,875 children aged 9-16 across five countries in Latin America - confirmed that the vaccine was safe, provided high protection against dengue haemorrhagic fever and cut by 80 percent the risk of hospitalization, the Paris-based company said on Wednesday.
It was 42.3 percent efficient in tackling serotype 2, one of the viral disease's four strains, compared to 35 percent in a previous Asian trial on some 10,000 children, a relatively weak rate that has puzzled scientists.
Overall, the findings were consistent and more reliable in the Latin American trial as it had twice as many patients as the Asian trial, said Nicholas Jackson, head of dengue research and development at Sanofi's vaccines unit Sanofi Pasteur.
"We're talking about different regions, different seasons, different demographics, and it's very important for a vaccine to perform consistently, so these results are extremely pleasing," he told Reuters in an interview.
The study was conducted in Brazil, Colombia, Mexico, Honduras and Puerto Rico. Sanofi will unveil its detailed findings at the American Society of Tropical Medicine and Hygiene's annual meeting at the beginning of November.
As in Asia, the results suggested 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 tourists.
However, given how the vaccine drastically reduces the most severe cases of dengue - by close to 90 percent - many countries and patients including tourists could see the point in using it, said Guillaume Leroy, head of Sanofi's dengue vaccine project.
He said Sanofi aimed to start filing regulatory applications for the vaccine early next year and ultimately reach out to 100 countries, but that it would target as a priority the countries where it led the trials.
Chief Executive Chris Viehbacher said in July that Mexico, Brazil and Colombia could be the first to market the vaccine.
The first batches - produced at a dedicated plant outside Lyon in southeastern France - will be ready next year and Sanofi aims to sell the first doses in the second half of 2015.
Some analysts believe the three-dose vaccine could bring in 1 billion euros ($1.31 billion) a year, significantly boosting Sanofi's vaccines business, which generated sales of 3.7 billion euros in 2013.
Leroy said Sanofi would apply for regulatory approval on a three-dose regimen to maximise the body's immune response to the jab, but declined to comment on pricing and sales estimates.
Nearly half the world's population is at risk of contracting dengue fever - also known as "breakbone fever" because of the severe pain it can cause. The disease infects some 100 million people each year, according to the World Health Organization, and some experts put the number at three times that level.
Most patients survive dengue but it kills an estimated 20,000 people each year, many of them children, and causes one hospitalisation every minute around the globe. (1$= 0.7618 euro) (1 US dollar = 0.7616 euro) (Editing by Keiron Henderson)