Essentially, herd immunity is this: you don't have to vaccinate everybody in a population to create an effective immunity within that entire population. You need to inoculate or have immunity in a certain percentage. That certain percentage depends on a lot of different factors, including the attack rate and such. Generally, the higher you get in terms of the population.... It could be entirely correct that if we get up to 50% or 60% of the population inoculated, that's enough, but right now we're not even close in terms of what people are saying they're going to accept.
On October 19th, 2009. See this statement in context.