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Perspective
Cost-effectiveness of West
Nile Virus Vaccination
Armineh Zohrabian,*
Edward B. Hayes,† and Lyle R. Petersen†
*Centers for Disease Control and Prevention, Atlanta, Georgia, USA; and
†Centers for Disease Control and Prevention, Fort Collins, Colorado, USA
Appendix 1. Cost-effectiveness
Formula
We used the following formula to calculate the cost per case of West
Nile virus (WNV) illness prevented:
 ,
where ACER is the average cost-effectiveness ratio, N is the size
of the population at risk for WNV illness, CV is the cost of
vaccination, and  and
 are the expected costs of a case of
WNV illness without and with vaccination estimated as the weighted average
cost of all health outcomes (the health outcomes and their corresponding
costs and probabilities are presented in the Appendix
Table). The denominator is the expected number of cases prevented
due to vaccination.
The overall sign of ACER is determined by the numerator. The denominator
is always positive because the expected number of symptomatic case-patients
without vaccination,  ,
is always higher than the expected number of symptomatic cases with vaccination,
 . When vaccination costs are
less than the expected savings due to vaccination, the numerator and ACER
will be positive, which indicates net savings due to vaccination.
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