Although reduction in transmission of an agent in the host population is an important goal of many vaccinations, suitable experimental methods to measure transmission have been lacking. Therefore, we designed and tested an animal experiment to quantify transmission among vaccinated and unvaccinated animals. We used Aujeszky's disease virus (ADV) in pigs, because a serological test was available to detect infection in vaccinated pigs and because vaccination against ADV will be used in an attempt to eliminate ADV from the Netherlands. Our experiments showed that vaccinating twice with vaccine 783 significantly reduces ADV transmission. In unvaccinated groups, the estimated maximum number of secondary cases per infectious individual, i.e. the basic reproduction ratio R0, was 10.0. In contrast, the reproduction ratio for the vaccinated groups R, i.e. the average number of secondary cases per infectious individual in a totally vaccinated population, was 0.5. These results show that it is possible to measure transmission experimentally. Therefore, such measurements should be obtained for all vaccines that are intended to eliminate agents causing animal diseases, either on a single farm or in a whole country.