By Alan Mozes
In the small trial involving just 41 healthy volunteers, one dose of the "TV003" vaccine offered 100 percent protection against a particularly tricky strain of the disease that scientists initially thought might be outwitting the vaccine.
Coupled with earlier indications that the vaccine also offers strong protection against three other strains of dengue, the results bode well for ongoing efforts to control the most widespread mosquito-transmitted virus in the world, the researchers said.
"Development of vaccines for dengue has been complicated, since disease can be caused by any of four dengue virus serotypes [strains]," explained study author Dr. Beth Kirkpatrick, director of the Vaccine Testing Center in the department of medicine at the University of Vermont College of Medicine in Burlington.
And a truly effective vaccine must provide "equal protection against all four," she added. That's because if someone who has been sick with one strain of dengue gets infected with a different strain, the second strain will cause more serious illness, she explained.
Kirkpatrick described the current findings as "encouraging," though she stressed that more research, on a larger scale, will be needed to confirm the vaccine's promise.
The study was published online March 16 in the journal Science Translational Medicine.
Dengue strikes roughly 390 million people each year, primarily in tropical and sub-tropical environments, according to the study authors.
Most infections are actually mild or without symptoms, the researchers noted.
But upwards of 2 million of those infected end up with dengue hemorrhagic fever, the study authors said. Symptoms include a very high fever, severe headaches, muscle and joint pain, blood vessel leakage and circulatory failure, according to the U.S. Centers for Disease Control and Prevention. For roughly 25,000 patients a year, the disease is fatal.
The new vaccine covers all four strains of dengue. It was first tried out in early 2016, with availability confined to just three countries: Mexico, the Philippines and Brazil, according to the study authors.
Early tests suggested the vaccine triggered a robust immune response for three of the strains. But, it first looked as if the vaccine might be less effective at producing antibodies for the "dengue 2" strain.
Kirkpatrick's team decided to retest the vaccine, focusing not only immune responses but also on infection rates.
The researchers recruited 41 healthy American adults (average age of about 30). The researchers tested the vaccine on people in the United States because dengue doesn't often occur in the United States, which means study volunteers wouldn't have been infected with any of the strains in the past.
Just over half the group was vaccinated with a single dose of TV003, while the remainder was given a placebo vaccine.
A half-year later, all were exposed to a genetically modified version of the dengue 2 strain, the study said. The test strain was fashioned to prompt what Kirkpatrick described as only a "minimal health risk," meaning mild and almost symptomless infections.
None of the vaccinated patients developed rashes or reduced white blood cell counts, or showed any signs of virus in their blood, the study showed. By contrast, all of those given a placebo vaccine had the dengue 2 virus in their blood. Four out of five developed mild rashes, and one in five saw their white blood cell count drop, the researchers found.
They now plan to study the vaccine in countries where dengue is widespread.
The current findings are raising hopes not only in the battle against dengue, but also for efforts to get ahead of other major health concerns such as the Zika virus.
"The dengue virus is closely related to Zika virus," noted Kirkpatrick. "The team working on this dengue vaccine is now leveraging their experience in efforts to develop a Zika vaccine."
But Matthew Aliota, a research scientist in the department of pathobiological sciences in the School of Veterinary Medicine at the University of Wisconsin-Madison, cautions that "much more work still is needed" for the dengue vaccine, as well as any potential Zika vaccine.
"This study is promising," he said. "However, work will be required prior to widespread introduction to allow evaluation of efficacy and safety."
"This," he said, "takes time."