As West Nile Virus Cases Rise, Vaccine Still Far Off

Challenges for Phase 3 Trials

"Due to limited resources and the reduced disease incidence observed in prior years, other research and development vaccine projects that would protect against more prevalent diseases in the world have been prioritized," Dr. Dayan told Medscape Medical News.

At some point in the future, the vaccine may be reevaluated, but for now, ChimeriVax-WN02 is on hold indefinitely, he said.

Other efforts to develop vaccines are likely to face the similar challenges, according to Mike Diamond, MD, PhD.

"Given that West Nile virus is still a low-incidence disease, affecting probably only 1% to 2% of the population over 10 years, and only a small subset getting significant disease, many people will need to be vaccinated to show efficacy in a phase 3 trial," Dr. Diamond, a professor of internal medicine in the Departments of Medicine, Molecular Microbiology, and Pathology & Immunology at Washington University School of Medicine in St. Louis, Missouri, told Medscape Medical News.

Such a trial will also involve a substantial investment, he noted.

"This trial could cost a lot of money — many millions — and it is uncertain whether vaccine makers will be willing to support this. Maybe, but again, this has been a major issue," he said.