Another reason why it isn't engineered. A mutation is as likely to make it harmless as it is to make it more harmful.
Mutation is neutral, that means that 50% will be more deadly and 50% less deadly, but at a mutation rate of 1.5/10,000 per per nucleotide per infectious cycle, 50% of these mutation will be more deadly. And, since this virus has already mutated to become extremely deadly (i.e., 1918), the possibility of this super, possibly human-engineered, 4-virus combination (strain) to become extremely deadly again is greater than 0. How much greater, we don’t know with current information.