Allowing for Two Different Mechanisms by which Lesion Free Lung Lobes Occur in Pigs After an Experimental Challenge: Vaccine Performance Comparisons

To compare vaccine performances in a pig pneumonia vaccine trial it is necessary to account for two mechanisms by which lesion‐free lung lobes may occur in pigs that have been experimentally challenged. A challenge may not establish in a pig and therefore necessarily there will be no lobe lesions. On the other hand a challenge may be established but the vaccine may prevent lobe lesions from developing. Therefore a model is required that unravels the nexus between the two mechanisms of generating pigs without any lung lesions. Some models are proposed based on zero‐modified distributions. Vaccine performance may then be assessed using a number of criteria. Data from an actual pig pneumonia vaccine trial are used as an illustration.