New approaches to the assessment of vaccine herd protection in clinical trials.

Criteria for the introduction of new vaccines into routine public health practice are becoming increasingly stringent. For vaccines that are expensive and those that provide moderate protection, the ability to confer herd protection could be crucial to policy deliberations about vaccine introduction. Traditionally, herd protection has been assessed after a vaccine is introduced, delaying the availability of data on herd effects to inform decisions about vaccine introduction. New methodological developments now provide the possibility to assess herd protection before the introduction of a vaccine into public health programmes. One approach is a cluster-randomised trial, which allows assessment of herd protection in a way that minimises biases. Analysis of individually randomised trials by appropriately selected clusters created post hoc can also provide measurements of herd protection. Here we discuss the use of these designs, which can generate an improved evidence base at an early stage for making decisions about the introduction of new vaccines.