The world’s first vaccine against the dengue virus has been shown to protect against three of the four strains of the virus in a trial in Thailand, the vaccine’s maker announced this week. The vaccine actually generated antibody responses against all four strains of the virus, but for some reason, one strain was still able to infect children who received the vaccine, the company said, and scientists are now trying to figure out why. Meanwhile, a much larger trial involving 31,000 adults and children is now under way in 10 countries in Asia and Latin America.

As many as 3 billion people, mostly in equatorial regions, are vulnerable to infections by the virus, which is carried by mosquitoes. In the United States, it is common only in Puerto Rico and Hawaii, although there was a recent outbreak in Florida. An estimated 230 million people are infected each year. As many as 80% of those do not develop symptoms, but 2 million -- mostly children -- develop a severe form called dengue hemorrhagic fever. Symptoms include fever, headache and muscle and joint pains -- which have led to the alternative name, break-bone fever. Infections can be lethal. There is no specific treatment for infections and currently no preventive agent.

Dengue is an RNA virus of the same genus as the West Nile, yellow fever and St. Louis encephalitis viruses. There are four strains of the virus. Infection with one strain provides lifelong immunity against that strain, but only short-term immunity against the other three.

Sanofi Pasteur of Lyon, France has developed a live-attenuated virus vaccine that produces antibodies against all four strains of the dengue virus in preliminary studies. It is given in three doses six months apart. In a Phase II clinical trial in Thailand conducted in partnership with the Mahidol University, Sanofi said in a statement, the vaccine provided protection against three of the four strains of virus. Full details of the study will be released in a scientific paper in September.


Other companies are also working on similar vaccines, but Sanofi is thought to be several years ahead of them. The company sems to be confident of its success. It has invested $423 million to build a factory in France to produce the vaccine. Experts believe the vaccine could now go on the market as early as 2014.

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