Methods and apparatus for providing an Anti-Flooding Flow-Control mechanism suitable for use in defending against flooding network Denial-of-Service attacks is described. More particularly, the invention relates to the identification of particular genetic modifications of attenuated VSV that result in an increased yield of virus and an increase in stability of the attenuated strains for preparation of the immunogenic compositions. Changing conductance of the metastable switching synaptic device occurs by receiving opposite signed first and second voltage pulses at the metastable switching synaptic device where magnitude of the first voltage pulse and the second voltage pulse each are below a switching voltage magnitude threshold.