The latest in a series of failed developmental pursuits of an effective HIV vaccine comes from Merck, who has discontinued its STEP study for ineffectiveness. The announcement was made today by the co-sponsors of this clinical trial, Merck & Co., Inc., and the HIV Vaccine Trials Network (HVTN), which is funded by the National Institute of Allergy and Infectious Diseases (NIAID), part of the U.S. National Institutes of Health. The study evaluated two primary efficacy endpoints: whether the vaccine prevented HIV infection and whether the vaccine reduced the amount of virus in those who developed infection. Results suggest the vaccination may have made recipients more susceptible to infection, in the midst of one of the world's most rampant epidemics.
The Merck trials took place in 15 cities in the US, including Los Angeles, New York and Boston. Three Canadian cities and research sites throughout Peru, Brazil Haiti, the Dominican Republic and Jamaica also participated in the study. These trials began December 2004 and included 3,000 participants, determined to be high-risk for infection. Enrollment and vaccination in a second Phase II trial of this vaccine being conducted by the HVTN in South Africa called Phambili, and two additional Phase I trials, have been discontinued. The DSMB for the Phambili trial will evaluate the available data. Researchers in Soweto, Cape Town, Durban and two other sites began contacting South Africa's 801 trial participants on Tuesday, mainly by cellphone text message. The goal is to tell each one individually whether he or she had received a placebo or the vaccine, a process called "unblinding" the trial. Researchers are telling the roughly half who received the vaccine that it might have increased their risk of contracting HIV.
Among those who received at least two doses of the vaccine during the STEP study, 19 contracted HIV, compared to 11 of those given placebos. Therefore, the vaccine did not prevent infection in the 741 volunteers who received at least one dose of the three-dose vaccine series, 24 cases of HIV infection were observed, compare to 21 cases of HIV infection observed in 762 participants who received the placebo. Investigators insist the vaccine could not have caused infection as subjects were injected with a killed, adenovirus serotype 5 (Ad5) virus structured in the likeness of the HIV virus. The vaccine could have, however, caused immunological changes that made it easier for the virus to take hold during a later exposure. Investigators found RNA levels of vaccinated-infected individuals were higher than those in the placebo group, who were subsequently infected. "HIV RNA levels in the blood of infected individuals, the standard measure of ongoing HIV replication, were approximately 40,000 copies/mL in the vaccine group and approximately 37,000 copies/mL in the placebo group."1 Results suggest vaccinated subjects with a previous exposure to the virus's structure may acquire more copies of the of virus if infected.
Development of an effective AIDS vaccine remains one of the most challenging pursuits of modern medicine. One positive aspect of the Merck's vaccine (MRKAd5) research cannot be denied; it showed that the Phase IIb test-of-concept trial design can yield earlier results with fewer volunteers than a full Phase III trial. "[This] probably saved us all waiting another 2 or 3 years, and the cost," an investigator says. Such trials may be beneficial over the full 10,000-person Phase III trial as research toward the HIV vaccine continue.2
1. Vaccination and enrollment are discontinued in phase II trials of Merck's investigational HIV vaccine candidate. Available at: http://www.merck.com/newsroom/press_releases/research_and_development/2007_0921.html. Accessed October 28, 2007.
2. Disappointment dominated the responses to the halting of immunizations in the STEP trial, but many agree it could hold important lessons for future vaccine strategies. Available at: http://www.iavireport.org/Issues/Issue11-4/Analyze.asp. Accessed October 28, 2007.
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