Chinese health officials confirmed the first case of human-to-human transmission of bird flu.

According to Beijing’s Chinese Center for Disease Control, a 24-year-old man spread the disease to his 52-year-old father in China’s eastern province of Jiangsu. The health agency conducted field and laboratory tests on both men, as well as 91 people who had had close contact with them.

The report concluded that the 24-year-old son was exposed to H5N1 virus while visiting a poultry market six days before he fell ill. His father, who survived, had had substantial contact with him while caring for him in the hospital. The father was not exposed to birds or other sick individuals.

The report found that the two men were infected with almost genetically identical strains of the H5N1 virus. It also found that 91 people who had come into close contact with the men had not been infected with the virus.

“Limited, non-sustained person to person transmission of H5N1 virus probably occurred in this family cluster,” wrote researchers at Beijing’s Chinese Centre for Disease Control and Prevention in the medical journal The Lancet.

“There is no indication from this data that we are any nearer to a pandemic,” said Ian Jones, a professor of virology at the University of Reading, according to the Associated Press.

A genetic analysis of the Chinese case found no evidence to suggest the H5N1 virus had gained the ability to transmit itself from human to human. However, the report raises concern amid health experts who have long said that the H5N1 could mutate into a form that is more easily spread. So far, that has not happened. If the H5N1 bird flu virus does manage to mutate, a pandemic could take place, killing millions of people worldwide, experts fear.

“Whatever the underlying determinants, if we continue to experience widespread, uncontrolled outbreaks of H5N1 in poultry, the appearance of strains well adapted to human beings might just be matter of time,” Dr. Jeremy Farrar of Vietnam’s Hospital for Tropical Diseases said in The Lancet.

According to the World Health Organization, at least 238 people worldwide have died from bird flu since 2003. Indonesia alone has 107 deaths from the bird flu.