Swine flu has been recorded in every prefecture, with Yamagata on Tuesday becoming the last one to see an infection, the Health, Labor and Welfare Ministry said.

Four high school girls in Yamagata Prefecture as well as their teacher, a woman in her 30s, were confirmed to have the new H1N1 virus when they landed Tuesday at Narita International Airport on a flight from Bangkok, the ministry said.

None had serious symptoms and they returned home, the ministry said, adding they were asked to remain indoors.

The quarantine system for swine flu has been revised and patients are not required to be segregated.