Foot-and-mouth disease (FMD) is a highly contagious and economically significant viral disease of clovenhoofed
animals. Vaccination can be used to help restrict the spread of the infection, but evidence must be
provided to show that the infection has been eradicated in order to regain the FMD-free status. While
serological tests have been developed, which can identify animals that have been infected regardless of
vaccination status, it is vital to know the probable prevalence of herds with FMD carriers and the withinherd
prevalence of those carriers in order to design efficient post-epidemic surveillance strategies that
establish freedom from disease. Here, we present the results of a study to model the expected prevalence of
carriers after application of emergency vaccination and the impact of this on the sensitivity of test systems
for their detection. Results showed that the expected prevalence of carrier-containing herds after reactive
vaccination is likely to be very low, approximately 0.2%, and there will only be a small number of carriers,
most likely one, in the positive herds. Therefore, sensitivity for carrier detection can be optimized by
adopting an individual-based testing regime in which all animals in all vaccinated herds are tested and
positive animals rather than herds are culled.