Experts said the discovery is a disappointment to efforts to find a cure for the human immunodeficiency virus behind AIDS, but offers important new clues in the hunt for the disease's elusive hiding places.
Only one person is believed to have been cured of HIV. American Timothy Brown, who suffered from leukemia, received a bone marrow transplant from a rare donor resistant to HIV and has shown no sign of the virus for six years.
"The return of detectible levels of HIV in our patients is disappointing, but scientifically significant," said Timothy Heinrich, a physician-researcher in the Division of Infectious Diseases at Brigham and Women's Hospital in Boston.
"Through this research we have discovered the HIV reservoir is deeper and more persistent than previously known," he said in a statement sent to AFP.
Heinrich first shared the news with fellow researchers at an international AIDS conference in Miami, Florida on Thursday.
Both HIV-positive men received bone marrow transplants as treatment for a kind of blood cancer known as Hodgkin's lymphoma, one in 2008 and the other in 2010.
About eight months after their operations, HIV was no longer detectable.
The patients stayed on antiretroviral therapy for while, but decided to cease taking the drugs earlier this year.
In July, doctors announced early, encouraging results: one patient appeared HIV-free after being off the drugs for seven weeks and the other for 15 weeks.
But signs of HIV soon returned. They were found in the first patient 12 weeks after stopping therapy, and in the second patient 32 weeks after stopping therapy.
"Both patients have resumed therapy and are currently doing well," Heinrich said, adding that the patients do not wish to be identified in the media.
He said it was important to share the results with fellow researchers, because they signal "that there may be an important long-lived HIV reservoir outside the blood compartment," and current methods of probing for tiny signs of HIV may not be precise enough.
"We have demonstrated HIV can be reduced to undetectable levels by very sensitive research assays, and the virus persists," he said.
Unlike Brown, these patients' bone marrow transplants did not come from donors that carried a genetic resistance to HIV by lacking a CCR5 receptor, so they were not afforded protection against the virus by that mutation.
The two men received transplants from donors with the CCR5 receptor, which acts as a gateway allowing HIV to penetrate the cells.
Researchers had hoped that by continuing to treat the men with antiretroviral drugs during the transplant process and afterward, the medicine might have prevented the donor cells from becoming infected.
Instead, it appears the virus was undetectable in the blood but lurked elsewhere in the body, allowing it to return once repressive drug therapy was stopped.