United Press International - Wednesday, 11 July 2001
Michael Smith, UPI Science News

A vaccine using the common adenovirus has allowed rhesus macaques infected with a hybrid test virus to remain healthy, while control animals sickened and died, said Michael Robertson, director of clinical research for Merck Research Laboratories in Blue Bell, Penn.

Early human trials to test the safety of the adenovirus vaccine have begun in healthy volunteers, Robertson said. Studies also have been started in HIV-infected people in the hope that the vaccine will help them control the virus.

Robertson told the International AIDS Society Conference on Pathogenesis and Treatment of AIDS that the animal tests are promising but "until you do human trials you never know." HIV is the virus that causes acquired immune deficiency syndrome.

Unlike standard vaccines that protect against infection, Robertson said, the adenovirus vaccine allows infection but permits the animals to shrug off the illness. They continue to have low levels of the virus in their blood, but after more than a year of testing, they remain healthy and their immune systems have not been destroyed.

In a standard HIV infection, the virus levels in the blood continue to rise, while cells of the immune system die, leaving the patient open to other infections.

The virus used on the macaques was SHIV, a virulent hybrid of the simian immunodeficiency virus, or SIV, and HIV, Robertson said. The animals were vaccinated four times, with an adenovirus that was unable itself to cause disease. It also incorporated a complete copy of the HIV gene. Jose Gatell, chief of infectious diseases at the University of Barcelona and chairman of next year's World AIDS Congress, said the results are important for several reasons.

"It's the first time that a protective vaccine has demonstrated some relatively important degree of protection," Gatell said. But he echoed Robertson's caution that human trials are needed before anyone celebrates.

"A vaccine that maintains the virus at very low levels would be a very good thing," he said, "because we could do away with all the drugs." If using SHIV in macaques is a good imitation of what happens when humans are infected with HIV, then the results will be useful, he said, but no one knows how well the animal model mimics human infection. "It's the best we have," Gatell said.

He added it is important that a giant pharmaceutical company like Merck is putting effort into developing a vaccine that might be expected to cut into their sales of anti-HIV drugs.

Robertson said one possible problem with the vaccine is that many people already have immunity against the adenovirus, because it is very common. But in the macaques that problem, he said, can apparently be avoided by boosting the vaccine dose or by priming the animal with the DNA of the HIV gag gene.

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