  Cluster-randomized trials are often conducted to assess vaccine effects.
Defining estimands of interest before conducting a trial is integral to the
alignment between a study's objectives and the data to be collected and
analyzed. This paper considers estimands and estimators for overall, indirect,
and total vaccine effects in trials where clusters of individuals are
randomized to vaccine or control. The scenario is considered where individuals
self-select whether to participate in the trial and the outcome of interest is
measured on all individuals in each cluster. Unlike the overall, indirect, and
total effects, the direct effect of vaccination is shown in general not to be
estimable without further assumptions, such as no unmeasured confounding. An
illustrative example motivated by a cluster-randomized typhoid vaccine trial is
provided.
