Number-needed-to-vaccinate calculations: Fallacies associated with exclusion of transmission

► Number-needed-to-vaccinate calculations are common metrics of vaccine efficiency. ► Traditional NNV calculations ignore indirect “herd” effects. ► Exclusion of herd effects may result in extreme upward bias of NNV. ► Bias is most problematic for diseases with low R0 and partially effective vaccines. ► Traditional NNV calculations should be used sparingly and interpreted with caution.