Timing, Location of Outbreaks Inconsistent
The main reason, experts say, is the highly inconsistent nature of when and where West Nile virus outbreaks occur, and how severe they will be.
With no reliable predictors, efforts to obtain sufficient enrollment for the extensive clinical trials that will be necessary for either a vaccine or an antiviral treatment are very difficult, said Susan L. Hills, MD, a medical epidemiologist with the CDC's Division of Vector-Borne Diseases in Fort Collins, Colorado.
"Because of the unpredictable incidence of West Nile virus over time and place, it's very difficult to set up a clinical trial," she told Medscape Medical News.
"You can't predict when an outbreak might occur or what level of disease you might have, so it makes it very difficult to set up an efficacy study."
Texas is often a hot spot for West Nile virus — although the virus has been reported in 48 states this year, as many as 40% of cases were in Texas — yet in some years other states have higher levels.
The virus is transmitted from birds to mosquitoes and then to humans, and although there are many theories on the causes of outbreaks, ranging from drought conditions to hurricanes, the bottom line is that scientists don't have enough of an understanding of why outbreaks occur in some areas and not others to predict where the next may occur.
"Unfortunately, outbreaks of any mosquito-borne diseases are always very difficult to predict, and it's not clear why we're seeing what we're seeing this year," Dr. Hills said.
Medscape Medical News © 2012 WebMD, LLC
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Cite this: Nancy A Melville. As West Nile Virus Cases Rise, Vaccine Still Far Off - Medscape - Sep 17, 2012.