PROS1 and COVID-19: Previous antigen-engineering strategy re-occurred in NDV-based COVID-19 vaccines, including stabilizing S protein by the introduction of 6 prolines and adding TM and CT domains of NDV fusion protein to enhance the expression of S protein on the surface of the viral particles.165,179 Representatively, rNDV‐S was constructed by expressing S protein of SARS-CoV-2 based on NDV vector.