Shares of VaxGen ( VXGN) dropped nearly 6% Wednesday afternoon following the company's announcement that its experimental AIDS vaccine failed a clinical test in Thailand.

The Brisbane, Calif., biotechnology company said the vaccine failed both objectives of the test: prevention of HIV, the virus that causes AIDS, and slowing the progression of the disease among those who received the vaccine but later became infected with HIV.

"The outcome of this trial is one more reminder of how difficult it is to combat HIV and how important it is for the international public health community to redouble the effort to develop an effective vaccine," said Donald P. Francis, VaxGen's president. "Although we are disappointed with the outcome, VaxGen and our Thai collaborators have created a model that can be used around the world for advanced clinical research of an HIV vaccine."

During the trial, 105 volunteers who received a placebo became infected with HIV; 106 volunteers who received at least one injection of AIDSVAX B/E became infected, the company said. The trial was conducted at 17 clinical sites in the greater Bangkok area. The company said the volunteers received education and counseling every six months "to avoid behavior that could lead to HIV infection." The volunteers also were advised to assume that they may have received a placebo or that the vaccine might not be effective, the company said.

In recent trading, VaxGen was off 55 cents, or 5.6%, at $9.30 a share.

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