EXPLORE: A Randomized Clinical Trial of the Efficacy of a Behavioral Intervention to Prevent Acquisition of HIV among Men who have Sex with Men (MSM)


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Explore Primary Study Results


The Lancet, a prominent medical journal, has published the results from Explore. What follows on this web page is a summary of the Explore results and several study results documents, including the article from The Lancet, in downloadable PDF format.


The article reports that there were fewer new HIV infections in the intervention group compared with the control group. This difference was not statistically significant, however, and was not at the level of benefit targeted by the research team.


The results suggest that the intervention had a somewhat (at most) modest benefit in preventing HIV infection and decreasing high-risk behavior among participants compared with standard biannual counseling and testing, and that that benefit occurred early in follow-up (within the first 18 months).


The Explore intervention affected some important risk behaviors. For example, it significantly reduced the occurrence of unprotected receptive anal intercourse (i.e., being a "bottom") with HIV positive or unknown status partners by 20.5%.


Declines in HIV incidence and sexual risk were seen in both arms of the study. The full effect of the intervention may have been muted since participants in both arms received considerable attention over the course of the study to maintain involvement and retention. Thus, participation in the standard arm was not equivalent to "usual care" in which persons would voluntarily seek anonymous counseling and testing.


Based on the results, the EXPLORE intervention, as currently designed, should not be distributed for widespread implementation. Further analyses of the Explore data, such as whether or not the intervention was more beneficial for certain sub-populations, are ongoing to generate ideas to help develop more highly effective behavioral interventions.


Other Results


The Explore study has made several notable achievements and provided some valuable research lessons:

  • There continues to be a high incidence of HIV infection among MSM. The HIV incidence rate for Explore was 2.1 percent. During the course of the trial, there were 259 new cases of HIV. The incidence rate of 2.1 percent is higher than anticipated; researchers expected a rate of 1.55 percent when they were designing the study.

  • The study observed declines in some high-risk behaviors – that is, the methods by which people contract HIV – in both arms of the study.

  • Explore showed that it is possible to do large-scale behavioral intervention studies among MSM.

  • There was community participation and interest in the development, implementation and dissemination of results of the study.

  • Explore provided lessons about the recruitment of large groups of high-risk MSM. The study recruited 4,295 participants in total. The risk profile of the screened participants is discussed in the Explore baseline papers. A paper detailing the Explore recruitment process is in the works.

  • The study achieved groundbreaking retention rates for a behavioral study, with a participant retention rate of 87 percent (85 percent in the standard arm and 89 percent in the control arm) over 4 years. A paper discussing the lessons learned regarding retention is in progress.

Next Steps


There are still many questions to answer about both Explore and behavioral/MSM/HIV prevention research in general. Some topics that will be important as "next steps" are:

  • We need to understand more aspects about the study and study participants to help guide future interventions.

  • Are there sub-populations of MSM for whom the behavioral intervention was more beneficial?

  • How can we engage an even larger proportion of MSM in interventions?

  • How can we engage men of color and younger men in interventions?

  • What is the link between alcohol and drug use and high-risk sex?

  • How did risk behaviors/incidence change in the control arm?

  • Explore offers the opportunity for dialogue about the study results and what communities want to learn from the study.

Lancet Article (Primary Study Results Paper)


This article was published on July 3, 2004 by The Lancet. It is the official word on the Explore study results. The document is in PDF format and is downloadable free of charge for your own personal use only. The article is reprinted with permission from Elsevier (The Lancet, 2004, Vol 364, pp. 41-50).

Explore National Press Release


This is the official press release from the HPTN announcing the Explore results and the publication of the article in The Lancet.

Explore Executive Summary


This document summarizes the Explore results. It contains much of the same information that appears in the Lancet article.

Frequently Asked Questions


This document addresses some common questions about Explore and the study results in a question-and-answer format.