First, I believe that for all cancers where it is used, Keytruda only works in about 70-80% of those who quailify genetically.

I know that some Keytruda patients relapse because there is one living across the street from me. So maybe that is where the AE37 vaccine becomes useful - prevents relapse and possibly works for those 20-30% for whom it did not work initially. Just conjecturing.