Concern is rising in parts of the US that a West Nile fever outbreak could cause an epidemic worse than that experienced in 2004, due to a wet spring and diminishing immunity in the bird population. The increasing impact of the West Nile Virus since its arrival in the US in 2000 highlights the need for a preventative vaccine, with Acambis [ACM.L] leading the race to bring one to market.
The West Nile virus (WNV) is a flavivirus, transmitted to humans via the bite of an infected mosquito, but not from person-to-person. The virus also infects birds, which act as the natural reservoir of the virus and are responsible for wider geographical spread. Although infection with WNV is asymptomatic in up to 80% of those infected, 20% of individuals develop clinical manifestations, known as West Nile fever (WNF), which can last several weeks. In 1% of patients the virus enters the brain and leads to more severe disease, and affected individuals who survive often suffer from permanent neurological after-effects.
Currently, neither prophylaxis nor therapy is available, and prevention focuses on protection from mosquito bites, while treatment is limited to supportive care. However, the rapid spread of the disease has created an unmet need for a prophylactic vaccine, in particular for older and at-risk populations. The US National Institute of Allergy and Infectious Disease (NIAID), therefore, has been developing two candidate vaccines, known as WN/DEN4-3'delta30 and VRC-WNVDNA017-00-VP, and has also provided financial support for external vaccine development.
Acambis is also developing a prophylactic WNV vaccine, known as ChimeriVax-West Nile, a candidate chimeric vaccine that, similar to WN/DEN4-3'delta30, is based on a live attenuated vaccine. ChimeriVax-West Nile was the first WNV vaccine to enter human trials and is currently undergoing Phase I trials, with results expected this year.
The clinical head start is key to ensuring Acambis's vaccine will be the first-to-market, enabling positioning as the vaccine of choice for catch-up vaccination of the at-risk group. Although mass vaccination against WNF is unlikely, the market potential of a WNV vaccine for the at-risk population is substantial.
Assuming that vaccination would cover citizens over 50 years, with a 50% compliance rate, and that the potential price could be up to $20 per dose, meaning the market potential for a WNV vaccine could be in excess of $260 million per annum. Moreover, additional patient potential could be gained through vaccination of travelers, army personnel and those who work outdoors in endemic areas.