It is not the vaccination that loses effectiveness over time, it is the fact that the immune system "forgets" over time. Which is why they reccomend booster shots and that there are very few vaccines that are only given once. Catching diptheria naturally does not give you lifelong immunity so why would having a vaccination against it give you lifelong immunity? Actually having whooping cough does not give you lifelong immunity, nor will the vaccine, again why they reccomend you have regular booster shots. the same with tetanus. If the full strength wild strains wont give you life long protection you can't expect the vaccines to, it is an unfair ideal for them to live up to.
Well I would have to disagree, i have had chicken pox and rubella as a child and as i am pg i have just had my blood work done and all the markers say that i am immune - no booster needed, no additional exposure required. Most of these 'diseases' are NEVER forgotten by your bodies immune system, in fact some of them are stored else where in your body.