The health ministry said in a statement that it found a Tamiflu-resistant strain of the (A)H1N1 virus in a five-year-old boy, who needed repeated and doubled doses of Tamiflu to recover.
Another anti-viral drug, Relenza, proved effective in specimens taken from the child but he was too young to take the drug, it added.
The ministry urged people to report immediately to hospitals or health authorities if Tamiflu had no effect five straight days after the first dose.
South Korea's swine flu death toll has reached 104 but the spread of the disease is waning, according to the ministry. - AFP