Immunisation levels of 85% to 90% have still allowed outbreaks of measles and rubella. Which explanation is given for that failure?

1. Herd immunity holds that a resistant majority shields the few who remain susceptible, because the agent is unlikely to find them. 2. That argument assumes susceptible people are scattered evenly through the population. 3. In practice they cluster in subgroups defined by socioeconomic or cultural factors. 4. If the agent is introduced into one of those clusters, it finds susceptible people packed together and an outbreak follows. 5. Herd immunity does not require everyone to be immune, which is the whole point of the concept, so that option contradicts it. 6. No loss of vaccine potency at high coverage and no change in the agent is offered as the reason. _Source: CDC Principles of Epidemiology in Public Health Practice, 3rd Edition, section Implications for Public Health_