Researchers have developed a vaccine that protects mice against West Nile virus (WNV), according to a report published in the current edition of the Proceedings of the National Academy of Sciences. Tests of the vaccine in monkeys are slated to begin next month, and human trials could start before the year is out.

The mosquito-borne WNV, which in humans can cause potentially deadly encephalitis, first appeared in North America in 1999. Since then it has claimed the lives of seven people and made 80 others seriously ill. To develop the vaccine, Alexander Pletnev of the National Institute of Allergy and Infectious Diseases and his colleagues enlisted the help of WNV's close cousin, dengue virus, which does not attack the central nervous system. By substituting certain WNV genes with the dengue variety, the team created a hybrid virus that stimulates a strong immune response without infecting the brain. None of the mice that received the hybrid vaccine developed WNV when later exposed to it.

Because one of the dengue viruses used in the construction of the hybrid virus has already been shown to be safe in humans, the researchers hope that their vaccine will advance swiftly to human clinical trials. "We must remain vigilant and act quickly if we are to keep ahead of emerging and re-emerging infectious agents such as West Nile virus," says NIAID director Anthony Fauci. "Disease-causing microbes will continue to adapt and continue to thrive, so we cannot let down our guard."