What is it about HIV that makes developing a new vaccine so challenging?

QUESTION 5
Development of HIV vaccines and boosting innate immunity.
None of the clinical trials of vaccines against HIV have proven to have sufficient sensitivity or specificity to progress into the general population. Researchers have suggested that incorporating molecules that innate immune response like ligands for toll like receptors with the HIV antigens may enhance immunity and improve the vaccine efficacy.
a) What is it about HIV that makes developing a new vaccine so challenging?
b) How might targeting the innate immune response improve the adaptive response and
boost vaccine efficiency?

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