It's not that absolute, in the same way as vaccinating for polio didn't totally eradicate it the first year. If the people are not exposed to that strain of E. coli, obviously they can't become ill. If our vaccine reduces or ultimately stops cattle from shedding it, then you're going to get to that point. But it's not all on, all off; no vaccine is.
On May 27th, 2009. See this statement in context.