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February 25, 2008

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An Update Concerning the
HVTN 503/Phambili HIV Vaccine Study

In October 2007, NIAID and the HIV Vaccine Trials Network (HVTN) stopped immunizations and enrollment in the HVTN 503 (“Phambili”) HIV vaccine study in South Africa. The following are interim data concerning enrollment and the number of HIV infections in the Phambili study:

  • There were 801 participants in the Phambili study in October 2007 when immunizations and enrollment were stopped. This included 360 women and 441 men.
  • As of January 22, 2008, 11 confirmed HIV infections among the 801 participants had been reported. Seven of these infections were reported among those participants who received the vaccine. Four HIV infections were reported in the placebo group.
  • Among the 11 confirmed cases of HIV infection, eight were detected before the study was “unblinded”—that is, when investigators and study participants were told which participants received the vaccine and which received placebo. Two of the cases were detected post-unblinding, and the study team is working to clarify whether an additional HIV infection occurred before or after unblinding took place.
  • Six cases of HIV infection occurred among vaccine recipients with levels of antibodies against adenovirus type 5 (Ad5) above 18 units compared with three cases of infection among placebo recipients with similar Ad5 levels. Among vaccine recipients with no detectable antibodies to Ad5 at enrollment, there was one case of HIV infection among vaccine recipients and one case of HIV infection among placebo recipients.
  • Ten of the 11 confirmed cases of HIV infection were in women; one male placebo recipient with a high Ad5 level (>18 units) was infected.
  • Among the seven vaccine recipients who became HIV-infected, two received only one vaccination, four received two vaccinations, and one received all three vaccinations.

These data do not accurately reflect HIV incidence rates within the study because the duration of follow-up of the study volunteers varies considerably by individual and by group. In addition, behavioral risk factors, such as number of sexual partners, have not been accounted for.

To compare HIV incidence in the Phambili study to HIV incidence in the STEP (HVTN 502) HIV vaccine study will be difficult and may not be possible. The information collected after unblinding occurred may be considered biased by the fact that volunteers may have modified their behaviors based on their knowledge of whether they received the vaccine or the placebo. Further, without substantially more cases of HIV infection, it will not be possible to draw any scientifically valid conclusions about the vaccine’s effect on HIV acquisition.

NIAID and the HIV Vaccine Trials Network (HVTN) are committed to conducting further analyses to understand the findings from the HVTN 503 study and use them to inform future research on HIV vaccines. The study investigators will continue to follow all of the Phambili study volunteers and counsel them on how to reduce HIV exposure to limit the number of any new HIV infections.

Currently, other NIAID-sponsored vaccine studies involving Ad5 are paused or are proceeding only in individuals with Ad5 titers less than 18 units. For a complete listing of these studies, see Status of NIAID Adenovirus-based Vaccine Studies.

For more information about the Phambili and STEP HIV vaccine studies, see Immunizations Are Discontinued in Two HIV Vaccine Trials, An Update Regarding the HVTN 502 and HVTN 503 HIV Vaccine Trials and Questions and Answers: HVTN 502 and HVTN 503 HIV Vaccine Clinical Trials.

Media inquiries can be directed to the NIAID Office of Communications at 301-402-1663,

NIAID conducts and supports research—at NIH, throughout the United States, and worldwide—to study the causes of infectious and immune-mediated diseases, and to develop better means of preventing, diagnosing and treating these illnesses. News releases, fact sheets and other NIAID-related materials are available on the NIAID Web site at

About the National Institutes of Health (NIH): NIH, the nation's medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit

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Last Updated February 25, 2008