Re: Why does chickenpox vaccine confer only a 10-year immunity ?

I have done a bit of research on the chickenpox (also called varicella)
vaccine in current pediatric textbooks.  The vaccine was developed in 
Japan in the 1970s, and is a live attenuated strain of the virus.  This 
means that the injection contains a live virus which is unable to cause 
disease, but causes your immune system to mount a response which could 
later defend against infection with the real, infectious chickenpox 
virus.  According to my reference (listed below), there is no evidence 
that immunity wanes even as far out as 20 years (that is all of the data 
available at this time, because the vaccine hasn't been in use longer than 
this).  The current recommendations as far as I can tell do not call for 
repeated immunizations after 10 years, although as more data becomes 
available this could be recommended.  The vaccine is quite effective at 
preventing infection (100% effective in blocking serious disease, 86% 
effective in blocking any disease).

In my opinion (and the opinion of the American governing board of 
Pediatrics), you definitely did the right thing by immunizing your 
children against chickenpox.  It is believed that if 97% of all children 
are immunized, we could basically wipe out all cases of chickenpox within 
30 years.

Hope this helps.

Reference:

Hay, Jr., William W. et al. (eds). Current Pediatric Diagnosis and 
Treatment, 15th edition.  New York: Lange Medical Books/Mcgraw-Hill, 2001.