Vaccine a Disappointment
Merck Stops Trials On Once Promising HIV Vaccine
Merck has stopped clinical trials for an HIV vaccine that initially carried so much promise.
A clinical trial has shown that a promising HIV vaccine is ineffective. Merck, which developed the vaccine, was forced to cut its clinical trial short, when results showed that there was no significant difference in the prevention of HIV infection or in the severity of infection between the experimental and control groups. Of the 741 people receiving the vaccine, 24 were newly infected with HIV during the 13-month trial period. This is compared to the 21 newly infected individuals of the 762 people in the control group who received a placebo vaccine. The vaccine has proven promising enough in animal experiments and tests on small groups of people to deem a large clinical trial worthwhile, making these results even more disappointing.
Why is an AIDS vaccine so elusive?
The development of an effective HIV vaccine is a top priority of AIDS research. The value of such a vaccine would be immeasurable in any prevention program. An effective vaccine could alter the landscape of the AIDS world and be a decisive tool in curbing the AIDS pandemic.
However, to date, an AIDS vaccine has eluded the top researchers worldwide. There a number of factors which have allowed HIV to circumvent the immune system and potential vaccines. Firstly, HIV is highly mutable yielding a large number of variants. Specifically, the proteins in the outer envelope of the virus mutates rapidly, which allows the virus to escape detection by antibodies. Secondly, HIV integrates into the genome of T cells. This has a number of implications. Infection takes place via cell to cell transfer. The immune system is eventually compromised because of the damage to the T cells. And lastly, HIV is permanently maintained in tissues via the infected cells.
Why was the Merck vaccine so promising?
Unlike previous attempts at vaccines which focused on B-cell mediated immunity, the Merck vaccine targeted T-cell mediated immunity. During the initial infection, cytotoxic T cells play an important role in decreasing the viral load. However, once the immune system becomes compromised, it can no longer contain HIV and the viral load increases. Previous research in animal models has shown that vaccines which elicit strong T-cell response are able to contain the infection. However, HIV was never shown to be eliminated entirely. Rather, the vaccine was able to delay the progression of the disease by decreasing the total viral load. This also has implications for reducing secondary transmission.
Next steps
Merck will call an end to a clinical trial in South Africa as well and trial participants will continue to be monitored. As for research, there are still over thirty vaccines which continue to be studied and tested by other companies and involve different approaches.
