There are many different strains of the E.coli bacterium. It is naturally found in the intestines of all mammals (including us) and is generally necessary for proper digestion. E.coli O157:H7 is a particular strain that while unproblematic in cattle can cause digestive problems and serious kidney infection in humans, and sometimes death as a result. The most common mode of transmission is via cattle feces that infects meats during processing or dairy during milking. My understanding of the vaccine is that it reduces the occurance of only this particular strain. It kills that bad E.coli, not the good E.coli. Other modes of transmission are also possible, but they all essentially rely on feces contamination of food or water. E.coli O157:H7 was identified as the contaminant in the infamous Walkerton water supply which resulted in several deaths and thousands of infections. Cattle feces entered the water supply after storm overflows contaminated the muncipal supply, complicated by improper treatment of the water by local officials. New treatment guidelines emerged as a result of the tragedy. I have colleagues in veterinary epidemiology and my understanding is that this new vaccine is seen as a very good thing for food safety.