Previous immunity to adenovirus 5 (Ad5), a common cold virus, may explain why volunteers receiving Merck’s preventive HIV vaccine may have been more likely to become infected with HIV than those receiving placebo injections, suggest researchers associated with the international STEP study. The new information was reported yesterday at a special gathering of HIV Vaccine Trials Network (HVTN) researchers and community advocates in Seattle.

Ad5 was used in the vaccine being developed by Merck and studied by HVTN. The shell of the virus was used as a vector to deliver pieces of HIV’s genetic material to immune system cells.

According to researchers at the meeting in Seattle, 49 male participants who received the vaccine became infected with HIV, compared with 33 males who received the placebo. There was only one infection among female participants, so they were not included in the post-study analysis. The difference in infections among the men was particularly striking when they looked at participants whose immune systems were strongly reactive to the Ad5 vector: 21 of those vaccine recipients became infected with HIV compared with just 9 who received a placebo.

The researchers cautioned that the numbers are so small and the data so new, that it is way too early to tell whether this trend toward greater HIV susceptibility in the vaccine recipients is real, or merely coincidence. They promised to continue analyzing the data with speed and intensity in hopes of answering this question as soon as possible.