Study of Merck vaccine candidate is stopped in South Africa; trial is unblinded
Sponsors of a second study testing Merck & Co's MRK-Ad5 AIDS vaccine candidate in South Africa announced October 23 that they have permanently stopped immunizations in the Phase IIb trial. On September 21, sponsors of a sister trial in the Americas and Australia, called the STEP Study, discontinued immunizations after an interim analysis concluded that the product was not efficacious. At that time, vaccinations in the South Africa trial were suspended pending further analysis.
The decision to halt vaccinations in the South Africa study, called Phambili, was made on the recommendation of the trial's Data and Safety Monitoring Board (DSMB), an independent body charged with protecting the safety and rights of trial volunteers. The Phambili DSMB based its advice on analysis of the STEP data At the recommendation of the DSMB, the Phambili study will be unblinded, meaning volunteers will be told whether they received the vaccine or a placebo. Also on the DSMB's advice, volunteers will be counseled about the possibility that the vaccine candidate may have caused an increased susceptibility to acquiring HIV infection. STEP volunteers will receive this same counseling. Sponsors of the STEP trial are discussing when that study will be unblinded.
When the sponsors of the STEP Study announced the halting of vaccinations in that trial, they released data from 1,500 of the 3,000 volunteers enrolled in the trial. Among those who received at least one dose of vaccine, there were 24 cases of HIV infection, while there were 21 infections among participants who received at least one dose of placebo. Of those who received two shots, there were 19 cases of HIV infection in the vaccine group and 11 in the placebo group. This difference does not rise to the level of statistical significance.
Sponsors of the MRK-Ad5 trials are now conducting more detailed analyses of the results. More information is expected to be made available on November 7, at a special open session of the HIV Vaccine Trials Network (HVTN), a co-sponsor of the STEP trial, along with Merck and the National Institute of Allergy and Infectious Diseases (NIAID). The Phambili trial was overseen by these three parties and conducted by HVTN and the South African AIDS Vaccine Initiative.
For additional information, see STATEMENT: An Update Regarding the HVTN 502 and HVTN 503 HIV Vaccine Trials and the AVAC Statement on Phambili and STEP trials