Shares in Merck & Co have slipped 3.2% after the company revealed data from its recently-terminated project to develop a HIV vaccine which suggests that the jab actually made participants more susceptible to the virus.

The company’s announcement in September that it had halted clinical trials on V520 caused huge disappointment among researchers and AIDS/HIV groups. However, that despondency has turned to alarm as a result of Merck’s analysis of the STEP study, one of two Phase II trials that were being carried out.

STEP, which was co-sponsored by Merck, the National Institute of Allergy and Infectious Diseases (NIAID) and the HIV Vaccine Trials Network (HVTN), involved 3,000 HIV-negative volunteers. V520 was created using a mixture of three components, each made with a replication-defective version of one of the common cold viruses, adenovirus type 5 (Ad5), which served as a carrier for three synthetically-produced HIV genes.

At the time the study was stopped, Merck said 24 of 741 volunteers who got the vaccine in one segment of testing later developed HIV, while 21 out of 762 participants who received placebo also were infected. However, the latest data, presented at a HVTN conference in Seattle, showed that to date, 49 of 914 vaccinated men became infected with HIV, compared with 33 of the 922 men on placebo. Among the 778 male volunteers who had high immunity to Ad5, 21 of those vaccinated became infected with HIV compared with just nine cases of HIV in the placebo group.

This suggests that people vaccinated with V520 were more susceptible to infection but Merck is saying it just does not know if that is the case at the moment, while stressing that the jab itself cannot cause infection as the amount of HIV contained is far too small. In fact the firm said that “there are a number of confounding factors that make it very difficult to draw conclusions about this finding”.

Keith Gottesdiener, Merck’s vice president of vaccine and infectious disease clinical research, said that "the data from this trial are remarkably complex. We are analysing the data to try to determine if the results are due to immune responses induced by the vaccine, differences in study populations, or some other biological phenomenon we don't yet understand, or simply due to chance".

He added that "it will take some time before we understand why the vaccine did not work and why there was a trend toward more cases of infection in volunteers who received the vaccine”. Further analyses are being conducted and  a presentation is scheduled for February, 2008.

Anthony Fauci, director of the NIAID, said the new analyses “are both disappointing and puzzling” but “we must regroup and recommit ourselves to developing an HIV vaccine”. 40 million people are currently living with infection, and more than 25 million with AIDS have died. Last year alone, 4.3 million new HIV infections occurred worldwide.

The volunteers in the STEP study were warned to protect themselves from exposure to AIDS but were not told if they were administered the vaccine or placebo. A decision to unblind the study and inform the participants will be taken in the next 10 days.