A major study of an HIV vaccine has been canceled and its participants are being contacted by researches to discuss a slight chance that a small number of the study volunteers might be at higher risk of HIV infection.

Seattle and Vancouver, BC were two one of 20 cities in North America where volunteers were recruited to participate in the research, called the STEP Study.

The worldwide investigation, called the STEP Study, was conducted by the Seattle-based HIV Vaccine Trials Network (HVTN). The group has its headquarters at Fred Hutchinson Cancer Research Center and is sponsored by the National Institute of Allergy and Infectious Diseases (NIAID).

Studies of the vaccine made by Merck were stopped after a review of preliminary data indicated that the vaccine wasn't working. In its September 21 announcement of the study halt, NIAID said that "the vaccine cannot be shown in this trial to prevent HIV infection or affect the course of the disease in those who become infected with HIV."

According to the NIAID website, study volunteers have been or will soon be contacted.

Volunteers in the HVTN 502 study are being encouraged to return to their study sites for HIV risk-reduction counseling and protocol-related tests, so that investigators can fully evaluate the effects of the vaccine on HIV acquisition, including whether there is any increased susceptibility to acquisition of HIV infection among those who received the vaccine.

In a Tuesday press release, NIAID warned of an additional issue that came up after analysis of results from a similar study of the same vaccine. The second trial was done in Africa with slightly different methods.

Researchers looking at data from the African study concluded that some of those who were given the vaccine might actually have a slightly elevated risk of HIV infection. 

NIAID said Tuesday that the risk -- if any -- will be explained to STEP volunteers when they're contacted by study researchers. 

Researchers have been at pains to emphasize, "The vaccine itself cannot cause HIV infection."

Interim results of the STEP study were analyzed by the Data and Safety Monitoring Board (DSMB):

Based on its first evaluation of interim efficacy data, the DSMB found 24 cases of HIV infection among the 741 volunteers who received at least one dose of the investigational vaccine compared with 21 cases of HIV infection among the 762 volunteers who were vaccinated with placebo. In volunteers who received at least two vaccinations, the DSMB found 19 cases of HIV infection among the 672 volunteers who received the investigational vaccine and 11 instances of HIV infection among the 691 volunteers who received the placebo.

Because it was what they call a "double-blind" study, roughly half of the study subjects were given a placebo -- a safe, but fake injection -- instead of the actual vaccine. The widely-used study technique is done to reduce error, self-deception and other forms of bias that might creep into results. Neither the researchers nor the participants know who gets what until the study ends.

According to news stories, public health officials have taken different positions on how much information should now be given to the volunteers who participated in STEP. Some insist that the study subjects should be told if they received a placebo or the actual vaccine.

NIAID gently frames the debate this way:

Discussions are under way to define the details of the continued follow-up of the volunteers, including when the volunteers will be told whether they received the vaccine or placebo.

The recent results are a disappointment to scientists who once saw great hope for the vaccine.

In an October, 2005 story on the study, Seattle Times medical reporter Warren King cited scientists who called the vaccine "most promising in 20 years."

"We're really excited about it ... It's the one out in front," said Dr. Julie McElrath, network lab program director, head of the Seattle vaccine clinic and a Fred Hutchinson Cancer Research Center scientist.

If encouraging results continue, much larger tests could be launched within five years. If it is shown to be truly effective against the virus, it could be widely available to the public in about seven years, McElrath said.