Table 1: Factors that makes control by vaccines problematic [69].

(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