The 17-year-old girl had gone to Vancouver, Canada, with her family and returned to Hong Kong at the weekend via Seoul, South Korea.

Last Sunday she developed a sore throat and on Wednesday she attended the Secondary Five graduation ceremony at the United Christian College (Kowloon East).

She is a cousin of another confirmed case, a four-year-old boy, who had also traveled to Canada, Centre for Health Protection controller Thomas Tsang Ho- fai said yesterday.

"[The girl] developed a fever, sore throat and cough on the night of May 24," Tsang said.

She had to go to the hospital to be quarantined on Wednesday when her cousin became a confirmed case.

Though the girl had been wearing a mask while at school, the government recommended the school be closed for 14 days - from Thursday until June 10 - for thorough cleansing.

In the meantime, health authorities will be keeping an eye on those who had close contact with the 17-year-old, especially those from Secondary Five to Secondary Seven.

Tsang said health-care workers have been sent to the school to conduct contact tracing and other necessary work.

Tamiflu will be given to those in need, he said.

Up to 170 people, including at least 20 teachers and more than 100 students, have been affected by the case.

The school principal said all students have been notified and appropriate measures, including disinfection, will be taken while the school remains closed.

A notice has also been put on the school's website.

Tsang said there is no need to close all schools across the territory as the government was able to trace the source of transmission and it was not a sporadic case that had sprung up locally.

Hong Kong Subsidized Secondary School Council chairman Liu Ah-chuen said it is reasonable for only one school to be closed since the origin of the case is known and it is not a community outbreak.

"Other schools do not need to close. While the 14 days might cause some disruption for the students, the move is understandable as it involves the safety of community," he said.

Two other cases were confirmed as of 6pm yesterday, both returnees from North America.

One is a woman, 18, who returned from Toronto on May 25 and the other also a woman, 20, who had been studying in Massachusetts, and who too returned on May 25.

Those who have been in close contact with them have been reached, and the 20-year-old's father has developed upper respiratory symptoms, Tsang said, adding both cases were asymptomatic when they entered Hong Kong, which allowed them to pass border checks.

The total number of confirmed cases in Hong Kong as of 6pm yesterday was 15.

The government has altered its strategy with regard to school closures since the first outbreak of H1N1 in Mexico. Initially the protocol had been to close all schools in Hong Kong as soon as there was one confirmed case.