They can help your immune system get going faster and reducing the severity of the illness, but how much can vary.
That's a theraputic vaccine, which is not the same thing as a preventative vaccine. The concept for theraputic vaccines has been around for roughly 100 years... The practice, however, is a far different story, as theraputic vaccines are FAR from being developed for most diseases.
As I said, trying to use a preventative vaccine in a theraputic manner can be a VERY bad thing, and I seriously doubt there would be any doctor or pharmaceutical company out there that would sign on to trying an unproven, still-in-development preventative vaccine in a therapeutic manner, especially not on the frigging President of the United States.