This study was undertaken with the intention of determining if the vaccine could prevent HIV infection in HIV-negative individuals; reduce the amount of virus in those who do become HIV-infected during the study, or both. The benefit of the vaccine was that if found successful it would help in preventing HIV infection and whether the vaccine and or reduce the amount of virus in those who developed an infection. I any case the research study would backfire there was a risk o the participants getting infected therefore proper awareness was made to the members and had a perfect understanding of the entire process. (White)
Throughout the trial, all participants received HIV risk-reduction counseling and various supplies, such as condoms, and information to help them avoid HIV infection. Initially, the study only enrolled individuals with low pre-existing antibodies to adenovirus type 5 (Ad 5) based on earlier indications that the vaccine would have the greatest potency for these persons; however, subsequent studies with the same vaccine lead to an amendment that opened participation to anyone who met the study’s inclusion criteria irrespective of pre-existing levels of adenovirus antibodies.
The phase 2b `test-of-concept` study was designed to test Merck’s candidate HIV vaccine MRK Ads trivalent vaccine. The study was testing Merck’s candidate vaccine, the MRKAd5 HIV-1 gag, pol, nef trivalent vaccine, which is based on a weakened adenovirus (adenovirus type 5), a common virus that normally causes upper respiratory infections, such as the common cold, but that has been altered to render it unable to replicate. The vaccine is a mixture of three weakened adenoviruses that act as vectors, or carriers, for efficiently transporting into the body and presenting to the immune system three HIV proteins: gag, pol and nef. (White)
Initially, the participants in the study were HIV negative volunteers between the ages of 18 and 45years and are at a high risk of being infected. They all were screened to check the eligibility of the participants, and they made an informed consent, and the researchers went ahead to administer the injections. One of the ethical issues surrounding this study is that the vaccines were administered without any indication of the exact individual with whom a particular vaccine was administered. The trial had to be discontinued as because the trial would not meet the efficacy endpoints.
Out of the participants, some they moved from being uninfected to being infected. In one of the groups, 19 participants cases of HIV infection was observed and another11 participants who received the placebo. In certain STEP subgroups, there were more infections in volunteers who received the vaccine than in those who received placebo. A long-term follow-up analysis of participants in the Step Study, an international HIV-vaccine trial, has confirmed that certain subgroups of male study participants were at higher risk of becoming infected after receiving the experimental vaccine compared to those who received a placebo. The vaccine used in the study did not contain the HIV, but it did contain HIV genes that were delivered to cells using a vector that employed a type of cold virus known as adenovirus serotype 5. In scientific meetings, results from STEP were presented which showed that the vaccine neither prevented infection in uninfected volunteers nor reduced viral loads in those who became infected with HIV during the study (Hudson)

References
1. Hudson, CP. 'HIV-1 Step Study.' The Lancet 373.9666 (2009): 805-806. Web.
2.Merck & Co., Inc. HIV Vaccine R&D | Global Health Progress. (n.d.). Retrieved from http://www.globalhealthprogress.org/programs/merck-co-inc-hiv-vaccine-rd
3. White, Aaron M. 'HIV-1 Step Study'. The Lancet 373.9666 (2009): 805. Web.
4. Q & A: HVTN 502 and HVTN 503 HIV Vaccine Clinical Trials.(n.d.). Retrieved from http://www.niaid.nih.gov/news/qa/pages/step_qa.aspx