The AIDS-causing human immunodeficiency virus mutates in a way that can make it invisible to virus-fighting T-cells, complicating efforts to design an effective vaccine, researchers said Wednesday.

The envelope protein, or outer shell, of the virus contains amino acid chains that apparently change over time, evading T-cells that fight viruses, according to researchers Rodney Phillips and colleagues at Oxford University, England.Phillips studied the virus in six HIV-infected hemophiliacs. In one patient, changes in part of the envelope protein were observed in December 1989, less than six months after an initial analysis. Further changes were seen in a subsequent observation in February 1990.

Because of the mutations, the immune system T-cells did not recognize HIV as their target. Similar observations were made in two other patients, the researchers reported in the journal Nature.

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"We have detected variant viral sequences that were not recognized by (T-cells,)" said the researchers.

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