China and Hong Kong confirmed Wednesday their second separate cases of swine flu, both in passengers who flew in from North America, and stepped up the search for those who came into contact with them.
In Beijing, authorities said a student who recently returned from Canada and then took a train to the eastern province of Shandong despite feeling feverish, had tested positive for the virus.
Officials immediately started tracking down anyone who may have had contact with the 19-year-old, even as the search continued for passengers who were on the same flight as mainland China's first confirmed case of A(H1N1) flu.
In Hong Kong meanwhile, officials said a man aged 24 who had returned from the United States was being held in isolation in hospital.
A further six people who sat near him on his flight from San Francisco were quarantined, along with family members who met him at the airport Monday, said Thomas Tsang, controller of the Centre for Health Protection.
Another 45 people who sat near him have already left Hong Kong.
The latest case comes almost two weeks after a Mexican man with the virus landed in Hong Kong in what was Asia's first exposure to swine flu.
That triggered the high-profile quarantining in a city hotel of 350 guests and staff for a week.
Tsang said that a wider quarantine was not necessary after the latest case as the man had not moved widely among the population.
The infection was detected when he approached airport health officials on his arrival. Tsang said he was in a stable condition and had only shown mild flu symptoms.
Xinhua reported late Wednesday that a total of 79 passengers had entered China with fever or other flu-like symptoms on Monday and Tuesday, citing government figures.
It was not immediately clear whether any of them were suspected of having A(H1N1) or whether they had travelled with any of the confirmed swine flu sufferers. Xinhua said all were now either under observation or treatment.
Hong Kong and mainland China are both very nervous about infectious diseases following the outbreak of the SARS virus in 2003, which killed 300 people in the former and 350 in the latter.
More than 5,700 cases of A(H1N1) flu have been confirmed in 33 countries, according to the World Health Organization, which says 61 people have died.
In Beijing, the health ministry said China's second mainland case started feeling ill on Sunday, two days after flying in from Canada where he studies.
He boarded a train to Jinan city in Shandong province the next day despite a high fever, sore throat and headache, and alerted local authorities on the way who picked him up and took him to hospital.
Health authorities appealed to passengers who sat near him on his Air Canada flight and in the same train carriage to contact health departments.
But they warned that tracing train passengers would be more difficult than those on the plane as ID cards were not required in stations before boarding, the official Xinhua news agency said.
China's central television said 10 people who had been close to him on the flight had been traced, and Xinhua said another 60 were under quarantine in the hotel where he stayed for three days in Beijing.
The World Health Organization (WHO) said there was no cause for alarm.
"It's really important for the authorities to keep being vigilant, but also for people not to panic," said Vivian Tan, a WHO spokeswoman.
"We haven't yet seen a case of human-to-human transmission within China so it's encouraging, but a lot more has to be done to make sure that it doesn't spread."
China's first confirmed case on the mainland was a man who fell ill in the southwestern city of Chengdu after flying in from the United States via Tokyo and Beijing.
While most of those who may have come into contact with him have now been traced and placed under medical observation, authorities are still searching for 16 passengers who were on the same plane to Beijing.
He is in a stable condition, and according to Xinhua the second man is now also recovering.
It was unclear exactly how many people in mainland China were in quarantine, but a rough tally of official figures relating to the two cases found that at least 440 had been put in isolation in Beijing and other cities.