By Jacqueline Stenson

Source:Reuters

SAN DIEGO (Reuters Health) - An experimental vaccine to protect against West Nile virus has been shown to be highly effective in animals, including monkeys, and human studies are planned to get under way early next year, researchers reported here Friday.

Currently, the only way to prevent the mosquito-borne disease is to avoid the insects that transmit it. Mosquitoes contract the virus from infected birds. Most people who become infected experience no symptoms or only milder ones such as headache, fever and bodily pains. But many elderly people and others with weakened immune systems are at increased risk of encephalitis, an inflammation of the brain, from infection and would be good vaccine candidates.

At the annual meeting of the American Society for Microbiology, researchers released data indicating that the vaccine has been successful in mice, hamsters, horses and monkeys exposed to the virus. "All animals that were inoculated developed antibodies to the West Nile virus," said Dr. Thomas Monath, chief scientific officer at Acambis, Inc., a company in Cambridge, Massachusetts that is developing the vaccine.

When the animals, including 16 monkeys, were then exposed to the virus, none died or showed any signs of illness, Monath said.

But it remains to be seen whether the vaccine will work so well in people. "Will it be 100% effective in humans? We'll see," Monath told Reuters Health. "So far it looks 100% immunogenic in animals. There's nothing equivocal about this data. It's very impressive."

Monath said he doesn't anticipate many complications with the vaccine. "The side effects are likely to be minimal," and may include itching and redness at the injection site, mild headaches and low-grade fever, he said.

The vaccine is a modification of a long-used vaccine against the virus that causes yellow fever, a close cousin of the West Nile virus. "We use yellow fever vaccine as a vector for West Nile genes," Monath said. The intent is to prompt the body to develop immunity against the West Nile virus. And the hope is that like the yellow fever vaccine, the West Nile vaccine will confer lifetime immunity from a single shot, he said.

Animal research suggests that the yellow fever vaccine may offer some degree of protection against West Nile virus since the viruses are closely related, but the yellow fever vaccine is not expected to be highly protective against West Nile in humans.

Studies of the West Nile vaccine involving a small group of people are expected to begin the first quarter of next year, according to Monath.