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By Rob Waters

Nov. 16 (Bloomberg) -- The 2007 failure of Merck’s HIV vaccine in a worldwide clinical trial may be linked to previous exposure by many patients to a common cold virus used to carry the vaccine.

Researchers from Imperial College of London report that the immune systems of people who’d been exposed to a so-called adenovirus in the past “remembered” their exposure and produced a flood of infection-fighting cells after being injected with the vaccine. Those immune cells, called CD4, are the very cells that HIV enters and infects, making them plentiful targets.

While scientists who designed the trial had anticipated the cold virus may blunt the effectiveness of the vaccine, they hadn’t expected it could provide a stepping stone for HIV to enter cells and spread, said Abel Benlahrech, a London researcher who helped lead the study. More research is needed to find out if different viruses might be used to deliver vaccines to HIV patients without triggering the unwanted immune reaction, he said.

“We expected that pre-existing immunity would only render the vaccine less efficacious, and instead it increased the susceptibility of people to HIV,” Benlahrech said today in a telephone interview.

The immune cells migrate to mucus-secreting tissues of the body, which may have increased patients’ susceptibility to the HIV virus because those are the places where HIV infections tend to occur, Benlahrech said.

Thailand Trial

The recent trial in Thailand of two combined HIV vaccines that prevented some infections were delivered by a virus that infects birds, not humans. As a result, the people who received the bird virus had no immune response to it, Benlahrech said. That may have contributed to its success, he said.

The 3,000-patient Merck study, known as the Step trial, was halted in 2007 after 49 HIV infections occurred among those who got the vaccine, compared with 33 among those who got placebo. The difference was greatest among 778 men who had high levels of immunity to adenovirus before the trial began, with 21 in the vaccinated group compared with 9 among the unvaccinated.

Michael Robertson, director of vaccines clinical research for Merck, said it was too early to assess the results of the study, published today in the Proceedings of the National Academy of Sciences.

“It would be premature to suggest that this provides an explanation for the Step results, and the implications for other vaccines,” are unclear,” Robertson said in an e-mailed statement.

To contact the reporter on this story: Rob Waters in San Francisco at rwaters5@bloomberg.net.

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