It's human nature to hope for positive results after spending months or even years conducting a research study. In well-designed studies, however, scientists identify in advance the criteria for success, so their optimism won't color their conclusions when the study is completed.

It's fair to say that a study of an experimental vaccine for dengue fever just published in The Lancet was designed well. Unfortunately, the study failed to meet its primary criterion for success: significantly protecting school-age children from contracting the disease. So it's a bit hard to understand how a press release from the journal can characterize the results as a "dengue vaccine breakthrough."

Dengue fever is caused by a virus transmitted by mosquitoes. Some infections are mild, while others are severe and debilitating, and can lead to death. The World Health Organization estimates that 40 percent of the world's population is at risk for contracting the disease. There are no vaccines or particularly effective dengue treatments now, so finding a potent vaccine would be a big deal.

The study involved 4,002 schoolchildren in Thailand. The vaccine, made by Sanofi Pasteur, uses a modified form of the dengue virus that can provoke an immune response but can't cause disease. The ideal vaccine would be 100 percent effective in preventing cases of dengue. But here, the efficacy was only about 30 percent.

The good news is that the study showed that the vaccine is safe, and that it appears to work well against some strains of the dengue virus.

"This is an important first step in understanding the complexity of the immune response to a dengue vaccine and how to improve the vaccine to induce complete protection," Dr. Timothy Endy wrote in an email to Shots. Endy is a dengue researcher at Upstate Medical University in Syracuse, N.Y., who wasn't an author on this study.

Susan Watkins, a Sanofi spokeswoman, told Shots in an email that the company remains committed to developing the vaccine. Ongoing studies with 31,000 volunteers "will be important to document efficacy of the dengue vaccine candidate in a broader population and different epidemiological environments." The results are expected in 2014.

Vaccines aren't the only avenue scientists are pursuing to stop dengue fever.

Another approach is to reduce or eliminate the population of Aedes aegypti, the mosquito that transmits the dengue virus. Pesticides are one way to do that, but scientists from the British biotech company Oxitech have a different idea. They've developed a genetically modified male mosquito that carries a lethal gene. When the modified males mate with normal females, none of the offspring survive.

In a field trial conducted on Grand Cayman Island, reported today in the journal Nature Biotechnology, the Oxitec scientists say their approach was able to reduce the population of Aedes aegypti by about 80 percent.

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