New HIV Vaccine is a No-Go: What Went Wrong and What Will Happen Next?

Author:  Merolla Lisa
Date:  November 2007

The horrible impact of the AIDS epidemic is well known – 39 million people are living with HIV and three million people die from AIDS each year. And although many methods are used in the fight against HIV, ranging from condoms to a wide variety of drugs, the number of people with HIV is increasing. Many people think the solution may come in the form of a vaccine. However, the search for a vaccine recently hit a major obstacle, causing researchers to question their approach to HIV vaccinations.

This September, a review board stopped an international HIV vaccine clinical trial, which had been testing a vaccine developed by the pharmaceutical company Merck. This vaccine was hoped to prevent HIV infection, and also reduce the severity of HIV in those who did become infected. A review of the trial, however, showed that the vaccine was not effective. In fact, males who received the vaccine were more likely to get HIV than males who received the placebo. Researchers are trying to determine whether the vaccine increased people's susceptibility to HIV. They are still reviewing the data to figure out what caused the unwanted results.

The vaccine was based on adenoviruses, common viruses that cause upper respiratory infections like the common cold. The adenoviruses were altered so that they could not replicate, and then they were used as carriers to transport three HIV proteins into the body. These three HIV fragments could not combine to create a complete HIV structure; this rules out the possibility that the vaccine actually caused HIV infections in the trial participants.

These HIV proteins in the vaccine were supposed to activate the body's T-cells, a type of white blood cell in the immune system. The proteins would program the T-cells to recognize and kill any HIV-infected cell. That was the plan, but something went wrong.

The main clinical trial involving this vaccine was called the STEP study. Sponsored by Merck and the National Institute of Allergy and Infectious Diseases, it began back in December of 2004. The study included 3,000 adult participants from places in North America, South America, Australia and the Caribbean. Another clinical trial testing the vaccine, the Phambili Study, had just begun in February 2007 in South Africa.

Both these trials enrolled HIV-negative volunteers who had a high risk of HIV infection. They were randomly assigned to receive three injections of either the vaccine or a placebo. However, these trials were shut down this September when an independent Data and Safety Monitoring Board concluded that the vaccine was not working.

Out of the volunteers who had received at least one injection, forty-five people are now infected with HIV. Twenty-four of these people had received the vaccine, while twenty-one had received the placebo. Similarly, out of the volunteers who had received at least two injections, thirty people are now infected with HIV. Nineteen had received the vaccine, while eleven had received the placebo. Clearly the vaccine was ineffective, which spells trouble for AIDS vaccine research, much of which is also relying T-cells to do the job. But what researchers really want to know is whether the Merck vaccine increased the risk of contracting HIV.

Seth Berkley, president and CEO of the International AIDS Vaccine Initiative, emphasized the uncertainty involving these results. "We know the Merck vaccine didn't work. We don't know why. We know that more people who got the Merck candidate than got the placebo wound up becoming HIV infected. We don't know if that had anything to do with the candidate or not," he said in a statement November 7.

These are the questions that researchers are dealing with now, and there are many issues to sort through. First, there are several confounding variables that confuse things – some participants engaged in more risky behavior than others, and the participants all lived in different environments. In addition, the participants varied in their pre-existing immunity to the adenovirus used in the vaccine, adenovirus type 5 (Ad5), depending on how often they had previously been exposed to Ad5. Those with high adenovirus titers – meaning those with high levels of immunity to Ad5 – were more likely to contract the HIV virus during the study. Researchers are still working out what that correlation means.

This setback certainly does not mean the end for an HIV vaccine. "This setback should not and can not diminish our commitment to developing an effective HIV vaccine," Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases said. "What we learn from the STEP study will inform ongoing and future HIV vaccine research," he continued.

There are currently other HIV vaccinations being researched. For example, at the International AIDS Initiative, scientists are looking into vaccines that will activate antibodies, the part of the immune system that protects the body from foreign substances. They think that the antibody arm of the immune system will be more effective than the T-cells in preventing HIV.

In addition, a new study was released this month suggesting that HIV vaccines could be created using ancient viruses called the human endogenous retroviruses (HERV). HERV are present in every cell, but their activity is normally limited. HIV-infected cells, however, produce HERV. Therefore, a HIV vaccine could work by causing T-cells to targets cells expressing HERV. In other words, this vaccine would indirectly target HIV.

Those are some of the possibilities being explored as researchers try to determine what went wrong with the Merck vaccine. And no one is giving up just yet.

"Developing a truly effective, preventive vaccine has been, and will continue to be, an extremely challenging scientific goal," said Berkley. "But given the human stakes, we have no choice but to redouble our efforts to realize that goal."

Written by Lisa Merolla

Published by Pooja Ghatalia.