Many vaccines are based on weakened or killed versions of the disease-causing virus or bacteria against which the vaccine is designed to protect. This approach is too risky for a virus like HIV that mutates rapidly. Alternative vaccines are being developed in which fragments of HIV virus are carried in weakened versions of other viruses known as vectors. If these vectors could replicate in the body they could induce a greater immune response. This type of vector is being researched at present but has not yet been tested in clinical trials.