Horses had a West Nile virus (WNV) vaccine within roughly two years of the first discovery of the virus in the Western Hemisphere. It has taken a little longer to get a WNV human vaccine through the pipeline and to the American public. A representative of Acambis presented the results of the Phase I clinical trials of the company’s human WNV vaccine candidate, ChimeriVax-West Nile, at the National Foundation for Infectious Disease’s 8th Annual Conference on Vaccine Research May 11. Marketing for human vaccines takes longer due to increased safety and regulatory requirements for human vaccines.

The company is developing the vaccine using its proprietary ChimeriVax technology, which uses yellow fever vaccine to create chimeric viruses. The vaccine acts like a modified live virus, which provides very long-lasting, high levels of immunity, but is extremely impotent and cannot cause disease. The yellow fever vaccine is closely related to WNV and is one of the safest, most efficacious vaccines ever marketed, says Maureen Long, DVM, Dipl. ACVIM, assistant professor of large animal veterinary medicine at the University of Florida. 

In the study, 96% of subjects in a high-dose group and 100% in a low-dose group developed WNV-neutralizing antibodies 28 days after vaccination, and the vaccine elicited high antibody titers. Thomas Monath, MD, chief scientific officer of Acambis, says, “We are encouraged by the immune response generated by trial subjects who were vaccinated with a single inoculation of ChimeriVax-West Nile.”

An Acambis representative told The Horse that Phase II clinical trials of the human vaccine will be released in the near future