An experimental AIDS vaccine forged from the virus that causes the common cold blocked the disease from developing into its full-blown stage in laboratory monkeys, although it did not prevent the actual infection, a top scientist at Merck & Co. Inc. said on Monday.
The findings point to the potential of a future human AIDS vaccine able to curb the progression of the disease in those infected by HIV, the virus that causes AIDS, said John Shiver, director of vaccine research at Merck Research Laboratories.
"Given how dangerous an epidemic this is and the threat it poses to people in general, if our only vaccine is one that controls the infection but does not prevent it, then certainly that's a worthy goal until something comes along that's better," Shiver said in an interview.
He also said he could not rule out the possibility that a vaccine following the model used in the study could even protect against initial infection. Shiver said the monkeys were given massive doses of an extremely virulent strain of the virus that causes AIDS, dwarfing the viral exposure experienced by any person who becomes infected.
In a note of caution, he said that any human AIDS vaccine was still years away from being ready.
In a study carried out by Merck researchers, rhesus monkeys were infected with a cross between HIV and simian immunodeficiency virus (SIV), the version of the AIDS virus that can infect monkeys and apes. Three monkeys were given the vaccine that used a form of the cold-causing adenovirus that was crippled genetically so it would not reproduce and was made to carry a gene from the AIDS virus.
VACCINATED MONKEYS "PERFECTLY NORMAL"
Those three did not develop the damaging aspects of the disease in eight months of observation, suppressing the virus to the point that it no longer was discernible in the bloodstream, Shiver said.
"They seem to be perfectly normal - which does make sense, considering how low the amount of virus is in these animals now," he said. "People with those very low levels typically would not progress to AIDS if they maintained those levels."
Six other monkeys were given no vaccine, with five getting the disease and four of those being euthanized, he said.
Full-blown AIDS is the last stage of HIV infection. The virus damages cells in the immune system that normally fight off infections and diseases. Typically, HIV lives in an infected person's body for months or years before the appearance of any signs of illness.
The Merck team designed the vaccine to elicit so-called cell-mediated immunity - one of two types of protective responses that can be mounted by the immune system. The body generates cells, called T-lymphocytes, that carry out search and destroy missions for cells already infected by a virus.
The researchers said the objective was to stimulate a strong and very specific anti-HIV cellular immune response, as opposed to an antibody response to intercept viruses before they succeed in invading a host cell.
The Merck researchers tested five vaccines, with the one involving adenovirus producing the most promising results - not in preventing infection, but in impeding the progression of the disease.
"It certainly is very hopeful. A couple of years ago, there was nowhere near as much reason to feel this kind of hope," Shiver said. "You always have to temper that hope with the realization that it's difficult to translate animal-based studies into humans."
The research was discussed at a meeting of AIDS researchers in Colorado. It followed recent successes in a similar vein by scientists at Emory University and Harvard Medical School.