The failure of an experimental AIDS vaccine trial two years ago may have been caused by the common cold virus. Researchers suggest the failure of an investigational HIV vaccine in the STEP trial, was due to CD4 memory cell expansion, the result of prior exposure to the adenoviral vector i.e. the common cold virus used to deliver HIV antigens.

According to Steven Patterson, PhD, Imperial College London, England, and colleagues, the vaccine prompted CD4-positive T cells that are HIV's main cellular prey, to accumulate in the mucous membranes of patients who had previously been exposed to certain adenoviruses.

The findings published online in Proceedings of the National Academy of Sciences explain why HIV infection rates increased in STEP participants after being vaccinated, leading Patterson and colleagues to theorize CD4-positive cell accumulations in mucosal tissues increased patients' vulnerability to HIV.

The results have led researchers to believe adenoviral vectors should not be used in any vaccine for pathogens with a mucosal infection pathway.

Approximately half the adults in the developed world, including 90% of individuals in sub-Saharan Africa, where HIV is most prevalent have built up immunity to the Ad5 adenovirus that was used in the STEP vaccine.