In fact, such a tetravalent vaccine is in clinical trials, he said. Last month, Sanofi-Pasteur reported that its candidate vaccine, tested in almost 21,000 children in Latin America and the Caribbean, reduced the risk of disease by more than 60%, and the risk of dengue-related hospitalization by 80%. The results mirror those seen in an earlier Asian trial, the drug company noted.There has been no such progress on a human vaccine against West Nile virus, Dr. Davis said. So far this year, 1,585 cases in 47 states have been reported to the CDC, with the majority occurring in California (563), Colorado (108), and Texas (207). Of these, 917 (58%) were classified as neuroinvasive disease (such as meningitis or encephalitis). Thus far, 54 patients have died; 20 of these in California alone.
The California Department of Public Health has identified 3,282 positive mosquito samples as of Oct. 27. Director Dr. Ron Chapman said in a press statement that the state has never experienced such a severe outbreak.
“The proportion of mosquitoes infected with West Nile virus is at the highest level ever detected in California,” Dr. Chapman said. “We expect to see more people become infected as this is the time of year when the risk of infection is the highest.”
Like dengue, the majority of West Nile cases are asymptomatic, Dr. Davis noted. But in the absence of a robust immune response, the virus can cross the blood-brain barrier and initiate a devastating neuroinvasive illness than can cause a flaccid paralysis, seizures, and death.
Neuroinvasive West Nile carries many clinical and diagnostic similarities to polio, he said. Like polio, the limb weakness has a sudden onset and can be accompanied by pain, although the sensory neurons aren’t apparently involved. Imaging studies show the virus attacking at the anterior horns of the spinal cord; nerve conduction studies show a motor axonopathy, but without much demyelination. Survivors have a variable course of recovery; some may regain full strength of the affected limbs, but many retain persistent weakness.
There is no vaccine against the disease, although several are in early-stage development, including a live, attenuated chimeric vaccine that did well in a phase II, placebo-controlled trial. Within 28 days of vaccination, 98% of subjects experienced seroconversion.
Dr. Davis had no relevant conflicts of interest.
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