Researchers in the United States say some of the viruses being used in experimental AIDS vaccines may cause damage to the immune system.
They suggest such viruses exhaust key cells and should not be used on people until more research has been done.
The comments from Dr. Hildegund Ertl, director of the Wistar Institute Vaccine Center in Philadelphia will not be seen as encouraging and serve to further muddy the waters when it comes to HIV vaccines.
In such experimental research usually harmless viruses, called adeno-associated viruses (AAV), act as vectors, carrying genetic material from the AIDS virus into the body so that the immune system can recognize it and mount a fight against it.
Dr. Ertl says the AAV vaccines may themselves be doing damage as the immune cells become exhausted and a simple defence mechanism on the part of the T-cells kicks in when there is too much antigen for too long a time, and they turn themselves off.
Dr. Ertl's team found this was the case in a study in mice where the AAV vaccines directly interfered with immune cells called CD8 T-cells, the very same "killer" T-cells that a vaccine aims to muster to fight HIV.
The immune system uses antigens to recognize enemies such as bacteria and viruses and in the case of HIV, turned-off T-cells could leave a person more vulnerable than usual to infection.
The researchers say while AAVs do not cause disease and cannot even replicate on their own but 'piggyback' onto adenoviruses, which cause colds, or herpes viruses, they do persist in the body.
Ertl's research comes at a time when trials of an AIDS vaccine that used another virus, an adenovirus by vaccine maker Merck was halted in September as the adenovirus used in the vaccine may have somehow made patients more vulnerable to HIV infections.
The International AIDS Vaccine Initiative or IAVI which also used an adeno-associated virus in a trial of an AIDS vaccine in Belgium, Germany and India, and another in South Africa, Uganda and Zambia was also stopped in January and the group is no longer testing AAV vaccines.
However experts say Ertl's research should be regarded with caution as the study involved mice which were given the human equivalent of far higher doses of the vaccine.
The research is published in the Journal of Clinical Investigation.