Answer:
Diversity and strategies.
Step-by-step explanation:
The problem would be with a vaccine to HIV is that HIV virus has diversity as well as strategies of HIV virus adopted to avoid the effect of vaccine so it can't kill the HIV virus from the body so the virus cause infection to the cells of the body again and again. The diversification of HIV strain reduce the impact of vaccine on HIV virus whereas using different strategies by the HIV virus to avoid effects of vaccine.