It's far too early to know if the vaccine will ultimately work, but a federal health official said these first results were a good sign.

The mosquito-borne West Nile virus has infected more than 16,000 people and killed 684 since its arrival in the United States in 1999. Most West Nile infections are mild and don't cause any symptoms, but the most severe cases can involve paralysis or swelling of the brain and death.

Prevention efforts have focused largely on insecticide spraying, and health officials have been hopeful a vaccine could be developed.

The Acambis test sample was small and was only the first phase, designed to determine the vaccine's safety. A vaccine won't be ready for market for at least three years, but the early results are encouraging, Monath said in an interview Tuesday before results were announced at an infectious diseases conference in Baltimore Wednesday.

"There's a road ahead, but this is a great start," he said.

"If we see similar results going forward, we should have a vaccine that will protect against West Nile with a single dose."

Acambis' research on the vaccine began in 2000 with a $3 million grant from the U.S. National Institutes of Health. Testing on volunteers began in 2003.

Acambis created the vaccine by combining genes spliced from the West Nile virus with a licensed vaccine for yellow fever, a relative of West Nile disease. That allows the West Nile genes to be carried into the body on a vaccine that's proven safe. The virus then causes a weakened infection which the body fights off.

"That results in strong immunization in the same way as if you got naturally infected and recovered," Monath said.

In the second phase of testing, the sample size will increase, including the number of older adults. Testing increases to thousands of people by the third and final phase.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said it's too early to get overly excited about the West Nile vaccine, though he said he was impressed by the percentage of patients who showed high level of antibodies.

What was particularly encouraging is Acambis' early success using a proven vaccine to create a safe hybrid vaccine, he said.

"The potential for this is to shave off years in vaccine development," he said.