HIV does not have any enzymes to ensure that the replication of its genome is error free. Use this information to propose an explanation as to why it has been difficult for scientists to create a vaccine to HIV?

The human immunodeficiency viruses (HIV) belong to the category of retrovirus and leads to fatal condition to the affected individual due to its action on the immune system.

Explanation:

This virus is known to contain its own replication machinery that lacks the property of proof reading. But despite vaccine against its is not yet formulated. This is due to the ability of higher mutation rate of the associated virus. This genetic variability allows the spread of the virus all throughout the body but with specific or unique genetic composition.

As the genetic composition of the virus is different, it adds to the difficulty level in creation of the antiviral drug as the target for the drug will not be specified.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.