Researchers on Friday announced that two men previously thought to be “cured” of HIV, the virus that cause AIDS, are back on antiretroviral drugs.

According to The New York Times, researchers highlighted the cases at an international AIDS conference in July. Dr. Timothy Henrich explained then that the patients showed no trace of the virus following a risky medical procedure to replace their bone marrow. One of the men had been off antiretroviral drugs for seven weeks and the other for 15. (HIV normally returns within 4 weeks or so without treatment.)

Timothy Ray Brown, known as the “Berlin patient,” in 2008 underwent a similar treatment and today is considered the only adult patient believed to be cured of HIV.

Dr. Henrich said in a written statement that the relapses were “disappointing but scientifically significant.”

The men, Dr. Henrich added, are “in good health.”

Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, also said that he was “disappointed” with the results, adding that he believes either some old infected cells survived or they passed on their genetic material to new ones before they died.