South Korean Ministry of Welfare and Health on Friday confirmed four workers, engaged in slaughter of poultry when bird flu was spreading in the country two years ago, have been infected with the H5N1 strain of bird flu.

The four workers were confirmed cases of asymptomatic infectionof avian flu after series of blood tests made by (South) Korea Center for Disease Control and Prevention (KCDC ) and the United States Center for Disease Control and Prevention (CDC), said a statement released by the ministry.

However, the ministry stressed the four never showed symptoms of disease and the test result means the infected persons have been exposed to the avian flu virus.

"The results do not match the World Health Organization standard of confirmed human cases of avian flu infection," the statement said.

"There is no known public health risk of spreading the avian flu virus among humans...South Korea continues to be free of avianflu," underscored the statement, adding it is safe to eat the meant and eggs of poultry.

The first case of H5N1 bird flu hit South Korea in December 2003. A total of 19 separate outbreaks affected 392 poultry farms until March 2004, resulting in the culling of 5.28 million birds.

The KCDC conducted blood sample tests for the 318 poultry slaughters who were involving the slaughtering of the sick chickenand duck during that time.

The KCDC sent the four worker's sample to the CDC for further tests. The CDC sent back test results to South Korea on Thursday, confirming antibodies for bird flu were found in the four's samples.

The ministry also said the 318 workers, including the four, were injected with antiviral drug Tamiflu when they were engaging in the slaughter.

The statement added, in wake of the new development, the disease control authorities are investigating the possible risks that might have caused to the asymptomatic contraction.

Moreover, the disease control authorities are conducting antibody tests on serum samples that have been collected from the 1,600 local people who contacted disease poultry two years ago.

"It will take about three months to complete the tests," the statement said.

The H5N1 strain of the virus is one kind of highly infectious avian influenza, which has killed dozens of people in Asian countries since late 2003.