|MadSci Network: Immunology|
First of all, some background information about the chickenpox. This disease has nothing to do with chickens, it only makes humans sick. It is caused by the Varicella Zoster Virus (VZV), which is also known as the Herpes Zoster Virus because it can also cause Herpes Zoster.
In normal healthy humans that have suffered from chickenpox, especially when they had a lot of pox (200 to 500 is not uncommon), the virus will not cause chicken-pox a second time. Of course, there may be a few healthy people that get chicken-pox a second time, but this chance is much bigger in people suffering from AIDS, take drugs because of an organtransplant or have a less than normal immune system function.
Directly answering your question, immune cells will probably live long enough to protect you all of your life.
However, the virus which causes chickenpox can stay in the body after the pox are gone. It may then cause Herpes Zoster (shingles) which causes sever pain in the skin.
Now for the interesting part, what are the reasons for vaccination. Of course, chickenpox is not the worst disease to have and about 95% of all people in the western world have been ill with it. For other diseases, like polio or the now eredicated small-pox, there is more to gain.
However, people with a limited immune system can get very ill from infection. Moreover, 25 out of 100000 adult people die of chickenpox and the virus can cause problems when it infects pregnant women (defective babies).
Enough reasons for vaccination I'd think.
You may be talking about vaccination of people that already have had chickenpox. Since it is sometimes hard to check whether or not people really had chickenpox, organizations may decide to vaccinate all people in a certain group, without testing for immunity.
Information found, among other places, in the CDC brochure, downloadable HERE. It's in Acrobat Reader 3 format, but that reader can be downloaded for free.
Try the links in the MadSci Library for more information on Immunology.