[ejr]

Community density and population migration is a large factor in vaccine effectiveness. What most vaccines provide is really herd immunity. That is a disease, if it spreads, will be limited to a select few who aren't able to vaccinate or have compromised immune systems, preventing an outbreak of wide proportions among an otherwise healthy population.

It's a bit more complex than the widespread and largely unnecessary antibiotic abuse happening in livestock farms. Unnecessary because more sanitary and efficient methods exist for raising them that obviate their need. Disease can be contained by quarantine and preventative measures which isn't possible among a migratory population like humans.