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Sanofi's dengue fever vaccine trial flop rattles the experts

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Vaccine experts have been pouring over the results of Sanofi Pasteur's closely-watched study for a new dengue fever vaccine in an effort to understand what went wrong. When investigators launched the 4,000-patient study in Thailand they expected to get a 70% efficacy read-out. But the final figure wasn't half that--only 30.2%, missing the mark of statistical significance by a wide berth.

"This result knocked me off my chair," Scott Halstead, an adviser to the Dengue Vaccine Initiative, told Reuters. "This is a very sobering outcome and a lot of thought and further experimentation is going to have to be done in order to understand what happened. You can't be very confident that this vaccine is going to significantly reduce human illness."

The large Phase IIb study could have paved the way to regulatory approval. Instead, Sanofi will now have to wait for the data from two big Phase III trials with 31,000 people enrolled in Asia and Latin America.

The problem, investigators say, is that the vaccine provided protection against three of four dengue fever strains. But it failed to guard against a strain quite common in Thailand. And Sanofi investigators have been focusing on the upside, noting that they have achieved proof-of-concept data to demonstrate that a dengue fever vaccine is possible.

Several of the experts surveyed by The New York Times felt that the results were disappointing, representing a setback for the leading vaccine program in the field. But not everyone was downbeat.

"The fact that it only protects against three of the four viruses doesn't bother me at all," Duke-NUS's Duane J. Gubler told the Times, noting that a three out of four success rate might extend protection to a threatened population.

- here's the Reuters story
- and The New York Times report
- get more from FierceVaccines

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