Scientists at the National Institute of Allergy and Infectious Diseases made the vaccine by placing West Nile virus proteins into a modified virus that causes dengue fever.

Researchers injected 12 monkeys with the combined virus vaccine and injected eight other monkeys with either the West Nile virus or the dengue virus.

Six weeks later, all 20 monkeys were injected with West Nile virus. The dozen that received the combined virus vaccine developed antibodies that successfully protected the animals from West Nile.

In 2002, Illinois led the nation with 884 cases and 64 deaths. No human cases have been reported in Illinois this year.