Japanese authorities scrambled Saturday to track travelers who arrived on the same flight as three people diagnosed with the country's first confirmed cases of swine flu. Australia also joined the ranks of affected countries with its first confirmed case.

Authorities in Tokyo quarantined a high school teacher and two of his teenage students who returned Friday from a school trip to Canada on a flight from the U.S. after they tested positive at the airport. A lab at the National Institute of Infectious Diseases later confirmed they had the virus that has killed at least 47 people and sickened more than 3,100 in 30 countries, most in the U.S. and Mexico, Health and Welfare Minister Yoichi Masuzoe said.

Since the outbreak began last month, several countries, including Japan, have screened air travelers for flu symptoms. But news reports said as many as 11 people on the Northwest Airlines flight from Detroit that landed in Tokyo on Friday avoided those screenings. Another 13 continued on to other destinations, including Thailand and Taiwan.

Japan's Health Ministry said it was checking into the reports but could not immediately confirm them.

Masuzoe acknowledged it would be difficult to trace all those who came into contact with the three infected Japanese, who were isolated and recovering at a hospital near Narita International Airport.

Asia has been largely spared from the virus that continues to claim lives in worst-hit Mexico, which announced its 45th death and 159 more cases Friday even as it emerged from a national shutdown that closed schools and businesses and shuttered churches and soccer stadiums.

But a handful of cases have cropped up in the region, including in South Korea and Hong Kong. The Chinese territory quarantined more than 200 people in a hotel after confirming its first case in a guest a week ago. They were released on Friday, and many were unable to contain their delight as they poured from the building for the first time in seven days. One man hugged a police officer and broke into song.

Australia reported its first case on Saturday in a woman it said was no longer infectious. She first noticed her symptoms while traveling in the U.S., federal Health Minister Nicola Roxon told reporters.

New Zealand — the first country in the Asia-Pacific region to confirm cases — reported two more on Saturday for a total of seven. The two high school students returned last month from a school trip to Mexico — as had the five others diagnosed with the virus.

Masuzoe acknowledged it was unclear how effective the Japanese Health Ministry would be in tracking the other passengers on the flight because it included a number of tourists and non-Japanese whose personal information wasn't available.

"There are limitations to what we can do, but we will continue to monitor the situation and strengthen or relax such measures as needed," he told reporters.

Public broadcaster NHK TV urged people who were aboard the flight to call a special telephone number for consultations. So far, 49 have been traced and taken to a facility near the airport to be monitored for 10 days, officials said.

Six of those had a fever and cough and were transferred to a hospital for more tests, according to the Kyodo News Agency. The health ministry declined to confirm the report.

The three diagnosed with the new flu strain had visited Canada's Ontario province on a home-stay program with about 30 other students.