Objective - To evaluate the application of a vaccine in a population of animals. Sample Population - Field-trial data from the literature. Procedure - A spreadsheet simulation model was constructed to estimate the impact of a vaccination program, assuming various population sizes, transmission rates, and vaccine efficacies. Results - Total effectiveness (proportion of affected animals [ie, cases] avoided) increased with the vaccinated proportion of the population. However, with a highly efficacious vaccine, this relationship discontinued after a sufficient vaccination proportion was reached, reflecting herd immunity. Evaluation of a case study indicated that what may be considered a poor vaccine on the basis of its low efficacy may protect a substantial portion of the population if the vaccine is administered to a sufficient number of susceptible animals. Further investigation of a case study of horses indicated that evaluating a vaccine based solely on its efficacy could greatly underestimate its value. Conclusions and Clinical Relevance - When evaluating a vaccine applied to a population, in addition to the vaccine efficacy, the vaccination rate, cost of the vaccine, potential disease transmission rate, and number and cost of cases avoided must also be considered. Efficacy may underestimate vaccine value in terms of the reduction of indirect cases typically avoided when vaccination is applied in a population.
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