By Brenda Goodman, MA
WebMD Health News
Reviewed by Louise Chang, MD
If further studies confirm the findings, researchers say the new vaccine would be the first to fight the painful and sometimes deadly disease, which strikes more than 100 million people around the world each year.
Dengue viruses are carried by mosquitoes. They cause a high fever, severe headache, and muscle and joint pain. They also make people bleed more easily than normal. Infected people may bleed from the nose or gums, or they may bruise easily. Young children tend to get more severe cases than adults.
Dengue mostly strikes people in tropical areas. So it's largely been seen as a disease that has plagued developing countries. But in 2009, the illness popped up again in the U.S., infecting 90 people in southern Florida. It was the first time dengue fever had been seen in that state since 1934.
A New Vaccine, Decades in the Making
The new vaccine was tested in 4,000 school children in Thailand. The children got either three doses of vaccine or placebo shots spread six months apart. The vaccine was meant to spur immunity to all four of the viruses that cause dengue fever.
After two years, the number of dengue cases was virtually the same between the vaccine and comparison groups. That might have spelled failure for the vaccine, which had been decades in the making. But researchers got lucky. They happened to be testing their vaccine during what turned out to be a dengue epidemic.
"The high number of cases identified allowed us to look at [effectiveness] by [virus strain]," says Derek Wallace, MD, regional director of clinical development for Sanofi Pasteur in Singapore.
What they found was that vaccine offered 60% to 90% protection against three of the four dengue strains. The vaccine didn't seem to work against the virus responsible for most of the cases in the outbreak: DENV2.
Vaccine Is 'Two Steps Forward, One Step Back'
Wallace says his team is still trying to figure out why the vaccine didn't protect against DENV2.
"We are working to understand this finding," Wallace says. "We have no conclusions yet."
Serious side effects were uncommon and similar between the two groups. A larger trial of the vaccine, in 31,000 children in 10 countries, is ongoing and should help to answer some of those questions, he says.
The studies are sponsored by Sanofi Pasteur, the company making the vaccine. If the vaccine is successful, market analysts predict the company could see annual sales of $1 billion or more.
Outside experts say the vaccine isn't perfect, but it is an important step forward.
"We've never seen a vaccine that protected against any dengue in clinical efficacy trials," says Scott B. Halstead, MD, a senior scientific advisor for the Dengue Vaccine Initiative in Seoul, South Korea. "This is the first field test of a dengue vaccine."
Halstead has spent his career working on the dengue virus. He wrote a commentary on the study but was not involved in the new research.
But he also says that the failure to protect against DENV2 is important, since it accounts for about 40% of severe disease worldwide.
"The very virus this vaccine doesn't protect against is certainly the one you'd like to take out first," Halstead says. "This is two steps forward, one step back."
The study is published in the Lancet medical journal.
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