At this point, no one knows if it actually prevents HIV, just that it produces a immune response (which can be detected through blood tests) which indicates that it could.
In general, it is possible that an HIV vaccine could lead to remission of pre-existing infections, but it's impossible to know yet if this one would do that.
The vaccines that I'm aware of only prevent infection in people without HIV. You can look up the vaccines on clinicaltrials.gov and find out their efficacy in different populations.
