Table 1.

Factors that makes control by vaccines problematic [6–9].

(i)	Antigenic plasticity—all RNA viruses
(ii)	Possession of immune evasion maneuvers, for example, targeting antigen-presenting cells and lymphocytes
(iii)	Persistence and latency
(iv)	Entrance and major replication at mucosal surfaces
(v)	Poor or retarded immune induction
(vi)	Resistance to neutralization by antibody
(vii)	Immunity solely dependent on T cells