The world's largest ongoing HIV vaccine study has been expanded to consider multiple ways of vaccine might boost immune response. The US Institute of Allergy and Infectious Diseases (NIAID) is testing the safety and efficacy of a dual vaccine candidate.

It's called the HVTN 505 study and has been underway since June 2009. HVTN stands for the HIV Vaccine Trials Network, which is funded by the National Institutes of Health.

Mitchell Warren, head of AVAC, the AIDS Vaccine Advocacy Coalition, is following developments.

"The trial is being expanded both in terms of the number of participants, as well as what the trial is looking to answer."

Broader study

Enrollment is increasing from about 1,300 to 2,200 participants.

Originally, the study focused on whether the vaccine regimen could lower the amount of HIV in the blood if people were to become infected after being vaccinated. It did not focus on whether it could actually prevent infection. Now it does.

The change stems in part from encouraging results in 2009 of the RV-144 vaccine trial in Thailand. The trial proved protection is possible, although the effectiveness was too low to go to market.

Warren said, "Because of the Thai trial, what we saw in that vaccine actually preventing infection was, wow, we really need to then look differently at HVTN 505 and expand its ability to look at the question: could this vaccine actually also prevent infection, prevent acquisition of HIV?"

Learning more, doing better

In AIDS vaccine research in the 1980s and 90s, there really was little, if any, success in getting vaccine candidates to give a protective effect. So, the emphasis switched to whether vaccines could mitigate the infection.

Warren says there are two basicimmune system responses to infection.

"We have the antibody response, or the humoral immunity, which conceptually is the part of the body that could help prevent infection. And then we have what's called cell mediated immunity, which is the part of the immune system that will help modulate the disease if one were to be infected."

The HVTN 505 study will now try to activate both of those systems.

"It uses a prime boost combination of two different vaccines. One is a DNA vaccine that has snippets of HIV that can't cause HIV at all, but is meant to kind of prime the immune system. And then it has an Adeno 5 vaccine boost," said Warren.

Adeno 5 is actually a common cold virus. It's used as a vector or means of delivering the vaccine.

Choose carefully

All the participants in the study fall under the UNAIDS category of Men who have Sex with Men, or MSM. The latest US data show the group has a very high HIV infection rate, particularly young men of color.

Warren says it's important for medical studies to carefully choose a target group.

"Every trial is unique and to the degree that a trial can ask a very specific question in a very specific population, the better we can answer the question. If we just created a vaccine trial or a treatment trial and said anybody can be in our trial, we just want bodies, at the end of the day we would look at the answer and say we don't know what this means because these people were so different."

The HVTN 505 study is being conducted in 12 US cities. The executive director of the AIDS Vaccine Advocacy Coalition calls this the most exciting time in HIV vaccine research.

efficacy: capacity or power to produce a desired effect 功效，效能

immune system: 免疫系统

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（来源：VOA 编辑：实习生史莉萍）