We may not need a vaccine. This was about gene deletions as opposed to mutations. A while back there was an analysis of what each gene coded for. A number of them seemed to suppress natural responses to infection, so it seemed obvious that a less deadly version was just a few deletions away.

If infectiosness could be maintained while reducing death rate (a big if) then you'd have a naturally spreading vaccine. Theoretically we only need to quarantine the severe cases and let it evolve to a cold.