The ‘Muguga cocktail’ live vaccine, delivered by an infection and treatment protocol, has been widely deployed in Eastern, Central and Southern Africa to protect cattle against East Coast fever, caused by Theileria parva. The vaccine contains 3 component stocks (Muguga, Serengeti-transformed and Kiambu 5). In a previous study, parasites from vaccinated and unvaccinated animals were genotyped with a panel of micro- and minisatellite markers (Oura et al.2004a) and it was shown that only the Kiambu 5 stock establishes a long-term carrier state but there was no evidence for the transmission of this stock. Also parasite genotypes different from the 3 component vaccine stocks were identified in vaccinated animals. We now report a follow-up study on the same farm, some 4 years after the initial vaccination, aimed at establishing the source of the novel parasite genotypes identified in vaccinated cattle, determining the longevity of the carrier state established by the Kiambu 5 vaccine stock and re-examining whether vaccine transmission can occur over a longer time-scale. To do this, samples were taken from vaccinated and unvaccinated cattle and the parasites were genotyped with a series of micro- and minisatellite markers. The data indicate that the vaccine stabilates contain at least 6 parasite genotypes, the Kiambu 5 stock can be detected in many but not all vaccinated cattle for up to 4 years and can be transmitted to unvaccinated cattle which share grazing and that some of the vaccinated animals become infected with local genotypes without causing overt disease.