Five more South Koreans were infected with the H5N1 bird flu virus about three years ago but none of them developed any serious illnesses, officials said on Friday after recently completed testing on old samples.

South Korea, which did not have comprehensive testing at the time, sent samples of 318 poultry industry workers taken during an outbreak in late 2003 and early 2004 to the U.S. Centers for Disease Control and Prevention in 2005 for further examination.

Of those workers, four South Koreans were infected, the CDC has said. The government said in February the four did not develop major illnesses.

The results prompted South Korean health officials to send samples to the CDC from another 2,109 people and of these, five were also infected, the health agency said on Friday.

"The five did not develop major illnesses and have no strain to transmit bird flu," the Korea Center for Disease Control and Prevention said in a statement.

About 400,000 poultry at South Korean farms were infected by bird flu between December 2003 and March 2004, but no human cases were reported at that time.

All of the samples sent by South Korea to the United States were from people involved in the culling of about 5 million poultry.

There have been no reported cases of bird flu in South Korea since the 2003-2004 outbreak.