I am just speculating, but in many diseases like rabies and chickenpox you can give the vaccine after exposure to help the immune system fight the infection. It's because the period between exposure and symptoms is quite long, often more than a month for rabies for example.
HIV can stay in the body for years before developing AIDS. So you can actually recruit people that have been tested positive recently and have not developed the disease.
HIV can stay in the body for years before developing AIDS. So you can actually recruit people that have been tested positive recently and have not developed the disease.