Q. Why is it difficult to produce efficient vaccines against a viral infection like dengue and AIDS?

It is difficult to make vaccines for dengue because there are four different types of viruses that cause the disease (DEN1, DEN2, DEN3 and DEN4) and it is supposed that the protection against one of them aggravates the clinical manifestation when the person is later infected by the another dengue virus similarly, In the case of HIV, the production of a vaccine is difficult because the virus is highly evades and mutant the antibody action.