A worldwide team of scientists on Wednesday said the human immunodeficiency virus (HIV) was swiftly evolving to avoid the body's immune defenses, a phenomenon that adds to the challenge of crafting an AIDS vaccine.

Mutations in HIV enable it to rapidly sidestep genetic variations that offer a better natural shield against the pathogen, they said in a study released by the journal Nature.

"Even in the short time that HIV has been in the human population, it is doing an effective job of evading our best efforts at natural immune control of the virus," said Oxford University researcher Philip Goulder.