this virus as the death rate is not that high after all.  All survived people used their immune system to battle the virus and they succeeded.  

The vaccine is against the virus directly.  Basically the vaccine is a virus with reduced/abolished toxicity that gets injected into human body so that the body generates antibody to kill the real virus.  How to make "recombinant virus" holds the key.  Some vaccine will be more effective than others because the recombinant less toxic virus generated are different.