Two patients previously thought to be "cured" of HIV after undergoing bone marrow transplants are now seeing the return of the virus in their blood, U.S. doctors said Friday.

Timothy Henrich, a physician-researcher at the Boston Brigham and Women's Hospital, said the reemergence of the virus demonstrates that HIV reservoirs, latent cells carrying the virus, are "deeper and more persistent" than scientists had realized.

"The return of detectable levels of HIV in our patients is disappointing, but scientifically significant," Henrich said in a statement emailed to Xinhua.

"Through this research, we have discovered ... 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," he said.

The two HIV-positive patients, who do not want to be identified, received bone marrow transplants as part of treatment for Hodgkin's lymphoma, a cancer of the blood, respectively in 2008 and in 2010.

In both patients, HIV became undetectable approximately eight months after transplant, and in the spring of this year they agreed to cease antiretroviral therapy to test whether the transplant had eliminated the virus from their bodies.

In July, the researchers announced that the two have shown no signs of HIV after they were off antiretroviral therapy for 15 weeks and seven weeks, respectively.

But in August, the researchers detected HIV in one of the patients, who then resumed taking medication. The other opted to stay off the medicine but last month, after 32 weeks with no HIV detected, signs of the virus reemerged and the patient also resumed antiretroviral therapy.

"I should point out that we were always very careful to say that we were not claiming that our patients were cured, and that we would only know with long-term follow-up the extent to which their viral reservoirs were reduced by stem cell transplantation," Daniel Kuritzkes, a professor with Harvard Medical School working at the hospital, said in an interview with Xinhua.

The researchers said the findings can help scientists better understand how and where the virus persists, and the potential role of the immune system in clearing cells that silently harbor HIV.

"As it turned out, there must have been virus that still persisted in tissues or some location that we could not easily sample," Kuritzkes said, adding that the reoccurring virus might come from such a location.

According to the researchers, both patients are currently doing well on antiretroviral therapy.

"We have a lot more work to do in terms of analyzing samples we have collected from the patients and hope to make a more complete report ... soon," Kuritzkes said.

The researchers first presented the preliminary findings Thursday at an international conference of AIDS researchers in Florida. Henrich told the Boston Globe that they decided to release the results before analyzing all the data, so others in the field could know as soon as possible.

"We felt it would be scientifically unfair to not let people know how things are going, especially for potential patients," Henrich was quoted as saying.

Other researchers who heard the presentation said the results were disappointing but the Boston team's approach and data will dramatically advance strategies for battling HIV, the newspaper reported.

"This is certainly telling us a lot about persistence, what we need to do, and how low we need to drop the levels of HIV reservoirs in order to allow patients to achieve remission," Katherine Luzuriaga, a professor at University of Massachusetts Medical School, told the newspaper.

In October, Luzuriaga and colleagues reported that a 3-year-old girl in Mississippi, who was born with HIV and given antiretroviral drugs within 30 hours of birth, remains in remission despite being off medications for 18 months.

Currently, there is only one patient believed to have been cured of HIV. German doctors in 2009 reported that an American, Timothy Brown, was given a bone marrow transplant for leukemia and appeared to have been cured of HIV.

On Monday, U.S. President Barack Obama announced the United States will shift 100 million U.S. dollars into research efforts in the next three years aimed at curing AIDS.

A White House background statement said at that time that although several individuals appear recently to have been cured of HIV through aggressive therapy, these approaches are "too toxic or premature to apply beyond the research setting."

The statement added that these cases, however, provide clues to explore for possible new treatments and that this new investment will catalyze further research into this area, and could lead to a new generation of therapies to improve outcomes for people living with HIV.