Mathematical Model Suggests that a Partially Effective AIDS Vaccine Could Lead to Decline in HIV Transmission
An AIDS vaccine with an efficacy rate of as low as 25% could stem the spread of the disease in Africa "under certain circumstances," researchers said Friday at the conclusion of the AIDS Vaccine 2001 conference in Philadelphia. The Washington Post reports that Ronald Gray of Johns Hopkins University presented a mathematical model testing a hypothetical vaccine in the Rakai district of Uganda, "one of the worst-hit nations in Africa." According to this model, developed by Gray with help from researchers at Columbia University and Uganda's Makerere University, a vaccine that is 50% effective would have to be taken by everyone in the district in order to make the "reproductive number" of the epidemic -- the average number of people infected by an HIV-positive person before he or she dies -- less than one, which would "theoretical[ly]" lead transmission of the virus to "abate ... and eventually burn out." A vaccine that is 75% effective would have to be taken by 50% of the population to achieve the same result, and a 25% effective vaccine that was taken by 75% of the population would reverse the tide of transmission only if HIV-positive individuals received HIV treatment according to HHS guidelines. While HIV treatment such as this "does not exist in Africa now ... the possibility is not entirely out of the question," the Post reports. Gray cautioned that if people reacted to the development of a vaccine by "increasing their risky sexual behavior even a little," this effect would "completely wipe out the benefit of the vaccine."
The VaxGen Trials
The Post also examines two ongoing clinical trials of VaxGen's vaccine candidate AIDSVAX. The first is a $200 million study of 5,100 gay men and 300 women in the United States, Canada and the Netherlands who are "at high risk of acquiring HIV because of their sexual practices." Researchers involved have estimated that 1.5% of the participants -- two-thirds of whom are receiving the vaccine while the remainder receive a placebo -- will become infected with HIV each year "assuming those getting the vaccine are not protected by it." The trial is scheduled to end in late 2002, but "an independent monitoring panel will secretly review the data in November" to determine whether the vaccine is 60% or more effective. If the data show benefit, the study will be stopped, and those who received the placebo would be given the vaccine (Brown, Washington Post, 9/7). VaxGen President Donald Francis said, "I think [stopping the study early is] a high hurdle to achieve. I don't know how optimistic I would be" (Morgan, Reuters, 9/7). The Post reports that the FDA has "suggested" it might approve a vaccine with a demonstrated 30% efficacy rate. In the trial, preliminary evidence suggests that the presence of a vaccine has not led to increased risky behavior -- the percentage of gay men who reported having unprotected anal sex decreased from 60% at the start of the trial to 46% a year later, and the female participants also reported lower rates of unprotected sex. However, women who thought they had received placebos increased their risky behavior. "They have difficulty understanding the concept of placebo," researcher Bradford Bartholow said, adding, "Many women believe the placebo is the vaccine."
The second VaxGen trial involves 2,500 drug users in Bangkok, Thailand, and illustrates how researchers are attempting to address the issue of individuals who become HIV-positive during the study -- 300 people are expected to acquire the virus in the trial "assuming the vaccine doesn't work." VaxGen has purchased insurance policies for all of the study participants, and "people who become infected will be treated under the national [HIV treatment] guidelines set by the Thai government's health ministry," which starting next month will include three-drug combination therapy but will not include a protease inhibitor (Washington Post, 9/9).
Hope for Therapeutic Vaccines?
In other conference news, researchers presented evidence suggesting that vaccines currently in development could "also work as a treatment for those already infected [with HIV], boosting their immune system so they can temporarily stop taking AIDS drugs," the AP/Baltimore Sun reports. Experts, however, caution that "more experimentation" is needed to determine whether "therapeutic vaccines" would be safe and effective. Aventis Pasteur's ALVAC-HIV is "furthest along" of any dual vaccine candidate -- so far it has been given to more than 1,900 people, and Aventis will begin "large scale testing next summer." Dr. Martin Markowitz of the Aaron Diamond AIDS Research Center presented the initial results of the study, which examined 18 individuals "who began taking AIDS drugs within four months" of becoming infected with HIV. Thirteen of the subjects took four doses of the vaccine and then stopped taking their AIDS medications, while the other five quit their drug regimens without taking the vaccine. While the virus "returned to detectable levels in all the vaccinated volunteers," six of them saw their virus levels "f[a]ll back down to low levels ... compared with just one of those who was not vaccinated" (AP/Baltimore Sun, 9/9).
Other Vaccine News
Newspapers across the country this weekend continued to report on vaccine progress and the quest for effective treatment. Summaries of some of the stories appear below:
- Arizona Republic: Pivotal Research Centers in Peoria, Arizona, will be one of 10 sites across the country participating in Merck's first large-scale trial of a vaccine candidate that showed promising results in monkeys. The vaccine uses a genetically altered adenovirus, a virus responsible for common colds, to stimulate a cellular immune response. Merck has conducted four small tests of the vaccine in humans, but this trial will be its largest so far. The 167 volunteers will receive four injections and make 17 visits to the clinic during the duration of the 78-week study. Patients are not at risk of developing HIV from the vaccine, as it has been "engineered so it cannot give HIV to a healthy person" (Fehr-Snyder, Arizona Republic, 9/10).
- CNN Your Health: CNN medical correspondent Dr. Sanjay Gupta and reporter Christy Feig on Saturday examined the AIDSVAX trials, as well as the trials of a vaccine candidate based on the natural immunity to HIV found in a group of prostitutes in Nairobi, Kenya. Feig also interviewed several vaccine trial participants about their reasons for volunteering to test the vaccines. The show's transcript is available online (CNN Your Health, 9/8).
- Newsweek: In a Web exclusive, Newsweek interviewed Dr. Peggy Johnston, assistant director for AIDS vaccines at the National Institute of Allergy and Infectious Diseases, about the present outlook for an AIDS vaccine. The full interview is available online on the MSNBC Web site (Fording, Newsweek, 9/7).