There is a real life answer, of sorts to your perceptive question.
Your "5 out of 100" exists for people taking Herceptin having trouble with the drug that also targets the oncogene but there is an easy solution with Herceptin in the vast majority of cases.
Doctors can just take women with breast cancer off the drug if they do not die quickly like a very small number did.
They can't, however, somehow turn a vaccine off.
That is not a full answer since a vaccine is far more powerful than a normal drug but consider ADXS is now in the antigen business and does not need killer antigens with a likely huge surplus of other antigens.