Vaccine Development

Developing a vaccine for dengue has been especially challenging because it is caused by any one of four related viruses transmitted by mosquitoes; developing a vaccine against just one of the serotypes will not prevent infection from the other three.

An individual who becomes infected with one serotype will develop protection from subsequent infection of that same serotype. If that individual is later infected with another serotype, however, they will not only be at risk for infection, but the disease often will be more severe, Kochel said.

In addition, simply developing vaccines for each of the four serotypes and combining them is not likely to create a successful vaccine, according to Porter, who pointed out, “The interactions of the different four in the same test tube may only result in you getting immunity to dengue [virus type] 1 and dengue [virus type] 2. That’s why this takes a long time to develop.”

The formulation being tested in humans is a DNA vaccine, which uses genetically engineered DNA to produce an immunological response. If it proves successful, Porter said, the DNA vaccine would be easier to store in a deployed environment than a live-virus vaccine.

“It is easier to make and, so if you have a DNA vaccine against dengue that is protective, then we imagine it would be a little bit more conducive to a deployed environment. So, if you go to the desert or you are launching troops to a dengue-endemic area, it is not necessary to have a minus 80 freezer in the theater of operations. You can maybe just keep it in a [regular] refrigerator or in a cooler,” he said.

For this initial trial, 40 subjects were assigned to three dose groups: a low-dose TVDV without adjuvant, a low-dose TVDV with the adjuvant, or a high-dose TVDV with the adjuvant.

Porter emphasized that with Phase I, such as this one, the main objective is to make sure the vaccine is safe and then to examine immunogenicity. Participants will be followed for a year as researchers look at the safety and the durability of the immune responses.

“If we hit a home run, then dengue is such a problem globally [that] we anticipate attracting corporate interest in further developing the vaccine to go to Phase II and Phase III studies,” he said.

Developing a Vaccine

The Navy’s dengue vaccine candidate is only one of the dengue vaccines under development by the civilian sector and the military. Another vaccine candidate by Sanofi-Pasteur is considered a front-runner and entered its first Phase III clinical study in Australia in late 2010.

Even with a candidate vaccine in Phase III clinical trials, Kochel pointed out that DoD considers its work on a dengue vaccine critical, because it is not assured that any of the current vaccine candidates ultimately will be effective or the optimal solution for the military’s issues.

“The reason that DoD is still engaging in vaccine development is because, No. 1, you can never be sure that a product will actually make it to market, even when it is in clinical trials. …,” Kochel said. “Another concern about the [Sanofi] product is that it requires three doses, and the total immunization period is a year. So, we are hoping that our vaccine candidate will require less time to offer protective immunity, maybe two or three months instead of the 12 months.”

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