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A clinical trial was conducted during the summer of 2004 to evaluate the effects of vaccinating cattle against Escherichia coli on the probability of detecting E. coli O157:H7 in feces and colonization at the terminal rectum. The probability for vaccinated or nonvaccinated cattle to shed E. coli O157:H7 in feces was not significantly different. However, the probability for steers to be colonized by E. coli O157:H7 in the terminal rectum was greatly reduced for vaccinated (0.3%) compared with nonvaccinated (20.0%) steers. We concluded that the vaccine was effective at reducing colonization of E. coli O157:H7 at the terminal rectum of cattle.