Dengue fever vaccine remains elusive

A promising vaccine against dengue fever produced disappointing protection rates in a recent trial. The vaccine guarded against three of the four virus strains, or serotypes, that cause the disease, but did not block the fourth, reducing its effectiveness. Still, researchers maintain that the results represent progress.

The search for a vaccine for dengue — a tropical disease that causes a characteristic rash and can lead to internal bleeding — has become more pressing in recent years as disease rates have risen dramatically. Today dengue is endemic in 128 countries compared to nine countries before 1970, and about half a million people are hospitalized with the disease annually, according to the World Health Organization. More than 2.5 billion people worldwide are at risk.

“People in endemic regions, they really, really want the dengue vaccine and they want it now,” Anna Durbin, a member of the Dengue Vaccine Initiative at Johns Hopkins University in Baltimore said. “They have large epidemics [that] overwhelm their health care systems. We had hoped that a lot of questions would be answered with this trial, but now we have a lot more questions.”

Researchers working for the French vaccine company Sanofi Pasteur, originally predicted that their second round of human trials would show high rates of protection in the 2,699 Thai children they inoculated. The actual success rate, published in a Lancet article last month, was only 30.2 percent, too low to offer productive protection.

Although these results fell short of expectations, the trial was significant in helping to quell safety concerns about testing new vaccine formulas in humans. A patient who has had one serotype of dengue is protected against that strain for life, but can still get the disease again—and often this second infection is more severe. Researchers were concerned that the trial vaccine might offer the same type of protection, which could put the participants at risk for more dangerous symptoms if they ultimately got dengue. But the study found no evidence of more intense disease in participants who contracted dengue during testing.

“The study shows that a vaccine against dengue is possible,” said Derek Wallace, Sanofi Pasteur’s regional director of clinical development in Singapore. “The absence of safety concerns and the evidence of protection against serotypes 1, 3 and 4 were encouraging.”

Though researchers are reassured by the study’s safety findings, the pressure to actually produce a working vaccine has not diminished. Successful vaccines already exist for diseases closely related to dengue like yellow fever and Japanese encephalitis, but the disappointing protection rate in this study may signal that a truly effective vaccine for dengue is a long way off.