By Kate Kelland, Health and Science Correspondent

SEATTLE (Reuters) - For Joe Cohen, a GlaxoSmithKline research scientist who has spent 24 years trying to create the world's first malaria vaccine, Tuesday, October 18, 2011 goes down as a fabulous day.

"There were many ups and downs, and moments over the years when we thought 'Can we do it? Should we continue? Or is it really just too tough?," he told Reuters, as data showing the success of his RTS,S vaccine were unveiled at an international conference on malaria.

"But today I feel fabulous. This is a dream of any scientist -- to see your life's work actually translated into a medicine ... that can have this great impact on peoples' lives. How lucky am I?"

Final stage clinical trial data on RTS,S, also known as Mosquirix, showed it halved the risk of African children getting malaria, making it likely to become the world's first successful vaccine against the deadly disease.

While scientists say it is no "silver bullet" and will not end the mosquito-borne infection on its own, it is being hailed as a crucial weapon in the fight against malaria and one that could speed the path to eventual worldwide eradication.

Malaria is caused by a parasite carried in the saliva of mosquitoes. It kills more than 780,000 people per year, most of them babies or very young children inAfrica.

Cohen's vaccine goes to work at the point when the parasite enters the human bloodstream after a mosquito bite. By stimulating an immune response, it can prevent the parasite from maturing and multiplying in the liver.

Without that immune response, the parasite re-enters the bloodstream and infects red blood cells, leading to fever, body aches and, in some cases, death.

Although Cohen's scientific work has been largely in Belgium, where he runs a GSK laboratory, the final-stage trials of RTS,S were conducted in Africa, where malaria hits hardest.

With GSK working in partnership with the non-profit PATH Malaria Vaccine Initiative (MVI), the trials became Africa's largest-ever medical experiment as the vaccine was tested in around 16,000 children across seven countries.

Cohen said that if all goes to plan, RTS,S could be licensed and rolled out by 2015.

SOUL SEARCHING

As he looks back at the vaccine's long, slow development, the bearded 68-year-old molecular biologist laughs at how naive he was when he first agreed to take on the task.

It was April 1, 1987 when his boss at the drug company, then called Smith, Kline & French, asked him to be head of the malaria vaccine research program, just after an early-stage experimental vaccine and failed a test.

"Unfortunately, it was not a great success. Only one volunteer out of the several that were vaccinated was actually protected. So, after quite a bit of optimism, there was then quite a bit of soul searching," Cohen said.

"I did not actually know much about malaria, apart from about the enormous medical burden it represented. But I felt I was taking on an enormous scientific challenge, and that was exciting for a relatively young scientist."

Having come from academia and post-doctorate studies into what he said were "sometimes esoteric questions" of molecular biology, he was also attracted by the prospect of doing something "very meaningful" in terms of global health.

Getting on for a quarter of a century later, Cohen said he had "never dreamed it would take this long."

He was also careful to underline that this was a first step, as well as a world first. GSK, MVI and several other research groups and drug firms are already working on next generation vaccines and on other ways of making malaria shots they hope will better the roughly 50 percent success rate of RTS,S.

"The work is not over, that is for sure," Cohen said.