South Korea Tuesday confirmed its fourth case of swine flu, a Vietnamese woman who was in transit through the country's main international airport.
The Korea Centers for Disease Control and Prevention said the woman, who was returning home Sunday via Incheon International Airport after a trip to Seattle, is infected with the A(H1N1) influenza virus.
"She has been quarantined in a hospital near Seoul," a spokesman told AFP.
The 22-year-old arrived on a South Korean plane with 209 people aboard including 93 transit passengers.
She had planned to take a connecting flight to Vietnam but was held for observation after complaining of high fever.
"We're now checking some 100 other passengers who entered this country on the same flight," the spokesman said.
All three previous victims in the country were Koreans, who have recovered and been discharged from hospital.
The last case was confirmed on May 7, a woman who had returned from North America on the same flight as Seoul's first confirmed swine flu case.
South Korea has stepped up its inspections of inbound travellers. It has announced plans to stockpile enough Tamiflu and other anti-influenza drugs to treat five million people, twice the number at present.