Two HIV-infected men who were thought to be clear of the virus after having stem cell transplants have had the AIDS-causing pathogen return, their doctors said, scuppering hopes their cases might lead to a cure.

The two Boston men, who were also suffering from lymphoma, had no trace of the virus eight months after the transplants, researchers led by Timothy Henrich at Brigham and Womens Hospital said in July, though they said it was too early to say the men had been cured. The virus reappeared in one of the men in August and in the other in November, Henrich said in a statement after presenting the results at an AIDS meeting in Florida yesterday.

The setback leaves just two people known to have been cured of HIV infection: Timothy Ray Brown, a San Francisco man who has been clear of the virus since having a bone marrow transplant for leukemia in 2007, and an infant born with HIV in Mississippi in 2010 who was considered cured after intensive treatment shortly after birth.

The new findings are disappointing, but scientifically significant, Henrich said in a statement. We have discovered the HIV reservoir is deeper and more persistent than previously known and that our current standards of probing for HIV may not be sufficient to inform us if long-term HIV remission is possible if antiretroviral therapy is stopped.

While AIDS drugs such as Gilead Sciences Inc. (GILD)s Atripla reduce HIV to undetectable levels in the body, making it a chronic disease, they dont completely clear it. The virus hides in so-called reservoirs, where it switches off the normal process of replication, enabling it to avoid detection by the medicines.

Studies have shown that when patients who have the virus under control stop treatment, the latent HIV reactivates and comes roaring back, forcing victims to resume daily pill therapy.

The two Boston men had stopped treatment following their transplants without the virus rebounding immediately, suggesting they may have been cured. In one man, the virus reappeared 12 weeks after he stopped treatment, and after 32 weeks in the other man. Both have resumed therapy and are in good health, Henrich said.

To contact the reporter on this story: Simeon Bennett in Geneva at sbennett9@bloomberg.net

To contact the editor responsible for this story: Phil Serafino at pserafino@bloomberg.net

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HIV Returns in Two Men Thought Cured After Stem Cell Transplant