I think by any measure POLIO has been effectively eradicated - I don't think there is any debate about this.
Now I'm going to get a bit technical re FLU so bear with me.
The FLU virus has certain protein on it that change - its unlike most of the other viruses that we vaccinate against that are stable.
So every year we actually produce a slightly different vaccine, based on the predicted FLU virus (which is usually the virus from the previous Northern Hemisphere Winter) and the proteins on it.
We have not eroded our ability to maintain a healthy immune system - our immune system is very robust, so much so that most of the NEW anti cancer drugs today are actually designed to boost our own immune system to fight cancer.
