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

Gap-fill exercise

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It is difficult to vaccines against
dengue because are four different
types viruses that cause the
(DEN1, DEN2, DEN3 and ) and it is
supposed the protection against
one them aggravates the clinical
when the person is
infected by the another virus.

In the case HIV, the production of
vaccine is difficult because virus is
highly mutant evades the antibody
action.