Merck & Co has moved quickly to tell volunteers in the discontinued STEP study of V520, the firm's HIV vaccine, whether they received the jab or placebo.

The New Jersey-based firm, along with the co-sponsors of the trial – the National Institute of Allergy and Infectious Diseases (NIAID) and the HIV Vaccine Trials Network (HVTN) – are unblinding the study after further analysis of the data suggested that the vaccine actually made participants more susceptible to the virus. All of the 3,000 volunteers will be encouraged to continue to return to their study sites on a regular basis for ongoing risk reduction counselling and tests.

Merck stopped the STEP study in September, a move which provoked disappointment among researchers and AIDS/HIV groups, after data revealed that 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 less than a week ago, 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 adenovirus type 5, one of the common cold viruses which served as a carrier for three synthetically-produced HIV genes, 21 of those vaccinated became infected with HIV compared with just nine cases of HIV in the placebo group.

Merck is still puzzled as to why the vaccine did not work and why there was a trend toward more cases of infection in volunteers who received the V520. The decision to unblind was welcomed by charities such as the AIDS Vaccine Advocacy Coalition which had earlier said that “if there is the possibility” that “informing every participant about whether he or she received the placebo or the vaccine will provide an additional safeguard for participants, then this is the course of action that should be taken”. Further analyses are being conducted and a presentation is scheduled for February, 2008.