A young brother and sister who flew into Hong Kong from the United States were confirmed yesterday to be infected with swine flu, taking the number of cases in the city to nine. They are in stable condition in Princess Margaret Hospital. According to the Centre for Health Protection, they had only minor symptoms. The boy, aged four, and girl, who is about 18 months old, are the youngest to be infected since the first case was found in the city on May 1. It was also the second time that two cases of the flu have been confirmed in Hong Kong on the same day, after the first on Saturday. The centre is tracing about 50 passengers from the flight who sat within three rows of where they were sitting in row 41, said the centre's controller, Thomas Tsang Ho-fai. Crew members were also being traced. The two children boarded Cathay Pacific flight CX 841, code-shared by American Airlines AA 6073, with their parents in New York after a tour in the city, New Jersey and Massachusetts from May 7 to 23. They arrived in Hong Kong at 2pm on Sunday. The girl had had a runny nose since Saturday, the centre said, and the boy had shown similar symptoms after arriving in Hong Kong. They went through body temperature screening at the airport without being detected. The family left the airport on the Airport Express, then took a taxi to return to their home in Block 7 of Cavendish Heights in Perkins Road, Jardine's Lookout. The children had stayed at home until about 9am yesterday, when their mother and maid took them by taxi to a private clinic in Repulse Bay. After an examination they were taken to Queen Mary Hospital by ambulance. The centre has appealed for two cabbies involved to call its hotline on 2125 1111. The parents and maid were taken to Princess Margaret Hospital for observation last night but showed no symptoms. A doctor, two nurses and a receptionist at the clinic all wore masks. They did not show symptoms and would receive Tamiflu. Earlier, the Department of Health said 34 people had to undergo testing for swine flu. So far the centre has tested 442, with 400 proving negative.