Reduction in burden of illness: A new efficacy measure for prevention trials

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Abstract
A new efficacy measure is developed for use in prevention trials of interventions which may affect both disease incidence and disease severity. We assign a severity score to each incident case and sum severity scores over all incident cases within each treatment group to create a burden‐of‐illness score for each treatment group. Efficacy is evaluated by the difference between the burden‐of‐illness per randomized subject in the two randomized treatment groups. Since the numbers of summands in each burden‐of‐illness score is a random variable, standard methods of analysis are not directly applicable. The asymptotic distribution and sampling properties of the net reduction in the burden‐of‐illness score are derived for trials designed to stop either after a fixed length of follow‐up or after the occurrence of a fixed number of cases. We illustrate the method with data from a clinical trial of a human rotavirus vaccine.