Understanding pre-existing immunity - February 2005. What is pre-existing immunity to a vaccine vector?
When vaccines are made using common viruses to carry a segment of HIV there can be problems with trial participants who already have immunity to the common virus. This pre-existing immunity can interfere with the immune response to HIV. The fact sheet describes current trials using adenovirus vectors to carry HIV and discusses the problems of pre-existing immunity.
Vax Online Primer