Would it be possible to defend to these kinds of attacks by creating (and spreading) less lethal forms of the killer virus (assuming general capability to desig

Would it be possible to defend to these kinds of attacks by creating (and spreading) less lethal forms of the killer virus (assuming general capability to design viruses but not the immune system) and bring overall lethality down (similarly how MRSA spreads less when there are natural competitors present)? This would solve the offensive biotech / defensive biotech dilemma to a degree.