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LOS ANGELES — The French pharmaceutical company Sanofi said Monday that its experimental vaccine for dengue fever had succeeded in its first late-stage clinical trial. The results could help pave the way for introduction of the first vaccine to prevent a disease that afflicts as many as 100 million people a year.

Sanofi said the vaccine reduced the incidence of dengue fever by 56 percent in a trial involving more than 10,000 children in Southeast Asia. It said that was a big enough reduction to achieve the primary goal of the study. A news release announcing the results did not contain much more detail. The company said it was still analyzing the data and would present more details at a medical conference and in a peer-reviewed publication later this year.

“This achievement is the result of more than 20 years of work in the field of dengue, collaborating with investigators, volunteers, authorities, scientific experts and international organizations,” Olivier Charmeil, chief executive of Sanofi Pasteur, the vaccine arm of the company, said in a statement.

Dengue fever is caused by a virus transmitted by mosquitoes and affects people mainly in tropical and subtropical areas. The disease, also known as breakbone fever, can cause severe fever and intense joint and muscle pain. Serious cases can cause hemorrhagic fever, which is marked by bleeding and shock. These can be fatal, especially to children.

The vaccine is one of the most important products in Sanofi’s pipeline. Sanofi has been trying to compensate for the loss of patent protection and the subsequent start of generic competition to some of its big-selling drugs, particularly the anti-coagulant Plavix.

Sanofi’s effort is being closely watched because it is considerably ahead of others in developing a vaccine. And such a vaccine is needed because there is no specific treatment for dengue and no preventive approach other than controlling the mosquitoes that transmit the disease.

The results of the late-stage, or Phase 3, trial, follow disappointing results announced in 2012 for a smaller Phase 2, or mid-stage study involving 4,000 children in Thailand. In that study, the vaccine reduced dengue cases by only 30 percent, far worse than expected.

The company, however, called those results a success. It said that the vaccine had failed to protect against only one of the four subtypes of the virus. Yet that subtype was the most prevalent one in that region during the time of the study.

It is not clear yet how the 56 percent reduction in the new trial will be regarded by outside experts. But even a 50 percent reduction could save many lives.