Many HIV tests work not by checking a person's blood for HIV, but rather by determining whether the body's own antibodies against HIV are present in the blood. Most HIV vaccines are designed with the intent to promote the body's own production of antibodies which would combat HIV. Regardless of whether they are effective at stimulating the production of antibodies which fight HIV, they usually do cause the body to produce antibodies which standard HIV tests recognize as the antibodies which an HIV positive person would produce in response to HIV infection.
