There are other potential explanations and caveats. The nutritional and health status of animals might affect the development of protective immunity in response to vaccination. However, more than 97% of dogs sampled from Serengeti district developed strong antibody titres (.0.5 IU/ ml) in response to vaccination [32] , suggesting that these factors do not impair the efficacy of dog vaccination in rural Tanzania. In addition, numerous practicalities-such as occasional failures in the cold chain, improper vaccination of animals, mistaken registrations, etc.-will all reduce the level of population immunity below the estimated vaccination coverage. Furthermore, our observations and simulations confirm that small outbreaks may occur simply by chance even when coverage exceeds P crit [33] , and these are particularly likely when there is individual variation in transmission (Figure 4 ). Higher levels of coverage are therefore necessary to reduce the chance of outbreaks with greater certainty; especially where the risk from imported infections is highest ( Figure 4C ). This could be a concern if canine rabies were to be eliminated from domestic dog populations but continued to circulate in sympatric wildlife; however, canine rabies was successfully eliminated in Western Europe and North America despite the presence of wildlife hosts capable of transmission.