South Korean officials have announced that four poultry workers were infected with avian flu more than two years ago, but have never become ill. These are the first known cases in South Korea of the virus spreading to humans.

South Korean official stands guard at farm affected by bird flu in Asan, south of Seoul

On Friday, the head of South Korea's Center for Disease Control, Oh Dae-gyu, confirmed four people had tested positive for antibodies to the strain of avian flu known as H5N1.

Dr. Oh says, however, South Korea is not classifying the cases as a human outbreak of avian flu, because none of the four has shown any symptoms of the disease.

The South Korean authorities say the four poultry workers were infected between December 2003 and March 2004.

At the time, the workers were involved in slaughtering poultry as South Korea tried to stave off an avian flu outbreak. About 400,000 birds were infected.

Dr. Oh says it took so long to identify the cases because South Korea at the time lacked the facilities to detect H5N1 antibodies safely.

Dr. Oh says the four cases were detected when 11 suspicious blood samples from workers involved in the 2003-2004 poultry cull were sent to the United States for testing.

South Korean officials say 2,000 workers were involved in the cull and 1,600 have still to be tested. The officials say all of the workers were given the antiviral drug Tamiflu at the time and none have ever shown any symptoms.

The H5N1 flu strain has killed at least 90 people and devastated poultry stocks since it emerged in Asia in 2003. Health authorities believe so far humans have only been infected through close contact with live poultry. However, they fear the virus will evolve into a strain that can be easily passed between humans, sparking a pandemic.