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A clinical trial conducted during the summer of 2006 evaluated the effects of two- and three-dose regimens of an Escherichia coli O157 vaccine product on the probability of detecting E. coli O157:H7 in feces and on colonization of the terminal rectum. The three-dose regimen significantly reduced the probability for cattle to shed E. coli O157:H7 in feces by 63% compared to placebo treated cattle. A dose-effect was demonstrated because a two-dose regimen of the vaccine product was intermediate in effect. These results are consistent with previous estimates of vaccine efficacy against fecal shedding and agree with our previous finding that efficacy is related to the number of doses.