“…This attenuation is achieved either by continuous passage either in vitro or in vivo of the virulent agent by exposing it to unfavorable conditions or by means of genetic manipulation like mutagenesis (e.g., modifying the virus by deleting viral genes essential for replication, host-tropism, immune evasion or invasion or by codon de-optimization) [ 51 , 52 , 53 , 54 , 55 , 56 ]. Several successful human vaccines have been made using this live attenuated vaccine platform including the Bacillus Calmette-Guérin (BCG) vaccine for tuberculosis (TB), the Measles vaccine, oral polio vaccine (OPV), and live attenuated seasonal influenza vaccine [ 54 , 57 , 58 , 59 , 60 , 61 ]. Except for BCG and measles vaccines, an important feature of these live attenuated vaccines is that they can be administered by routes other than parenteral i.e., oral or intranasal routes.…”