E. coli in meat has become more widespread, and more dangerous strains are now becoming more common, for several reasons. Most cattle are fed antibiotics as a matter of course - the crowding and stress, and their diet, in the feedlot can predispose cattle to become ill. Confined feeding operations cannot work as well (profit-wise) without antibiotics. This routine administration of antibiotics can lead to the development or spread of new strains of disease. The digestive system of cattle is not well-suited to the diet they are fed in feedlots - cattle were not designed to be fed corn, not to mention the other things that are in their feed (some of which are so disgusting as to not bear thinking about). Cattle in feed lots shed much more E. coli in their feces than cattle which are grass-fed. And finally there's slaughter operations - the high-speed industrial processing inevitably leads to frequent contamination of the meat with waste materials.
Dangerous strains of E. coli exist in the food supply for one reason - our industrial, factory approach to food production, which is neither good for the animals subjected to it, or for our health - a vaccine is merely a band-aid and really solves nothing. Just my opinion!