To produce the efficient vaccines against dengue is some what
difficult because there are four different types of viruses (DEN1,
DEN2, DEN3 and DEN4) which causes dengue.
And there are facts to suggest that the protection against one
of them aggravates the clinical manifestation when the person is
later infected by another dengue virus.
To produce the efficient vaccines against AIDS is also
difficult as the virus is highly mutant and evades the effect of
antibodies.