Work is ongoing. The virus mutates easily, so any vaccine needs to target parts of the virus that cannot change, and this has been a challenge. I like the answer provided by anthony s. Fauci, M.D. Director, national institute of allergy and infectious diseases national institutes of health on may 12, 2012: http://www.Niaid.Nih.Gov/news/newsreleases/2012/pages/hvad2012.Aspx.
Difficult. Hiv virus is a difficult organism- has several different proteins based on which vaccines need to be created. Hiv virus has a tendency to mutate making it difficult for vaccines to work. However monoclonal antibodies show some promise. Still research ongoing - hopefully we will have a breakthrough.