Expected values of the Direct, Indirect and Total efficacies of vaccination using a two-level design with 50% of herds vaccinated at 25% and of herds as unvaccinated whole-herd controls. We explore a range of assumed vaccine-induced reductions in susceptibility (A,B) , (C,D) , (E,F) ) and infectiousness (Linetype, 0, 30, 60, 90%). The expected effect sizes (A,C,E) are illustrated by the median of the posterior predictive distribution for each measure. Power (B,D,F) is calculated using a classical hypothesis test on the relative risk of infection (RR) in vaccinated compared to unvaccinated animals (described fully in Materials and methods section). Note that in this design, Direct Efficacy can be estimated relative to either within-herd (WH) or between-herd (BH) control animals. 100 herds provides power to estimate a protective direct efficacy for a true efficacy of 60%, however herds are required to achieve an 80% power to estimate a true efficacy of 30%. For the SOR model, the indirect efficacy is predicted to be ~10% as a consequence of the greater instantaneous number of infectious animals within herds as compared to the SORI model. This predicted effect size is still small and as such there is still only a 70% power of estimating a positive indirect efficacy for a study size of 300 herds. Power to estimate the Total Efficacy is likewise increased with a chance of estimating a positive Total Efficacy from 300 herds.