All passengers who were on a Mexico City-Shanghai flight with a Mexican national who was later diagnosed with A (H1N1) influenza have been located, health authorities said yesterday.

Of them, all on the Chinese mainland have been placed under quarantine. None of them had shown any flu symptoms such as fever by noon yesterday, the Ministry of Health said in a statement.

The 25-year-old Mexican arrived in Shanghai on Thursday aboard Aero Mexico flight 098, which carried 176 passengers and 13 crew. He then took another flight to Hong Kong after a short stopover.

The ministry requires all passengers who were on board with the Mexican to be quarantined for seven days as a precaution.

The ministry did not reveal the locations of the passengers, but at least seven mainland provinces and municipalities have reported placing AM098 passengers under quarantine. They are Beijing, Shanghai, Guangdong, Jiangsu, Hebei, Zhejiang and Sichuan, according to local media reports.

Shanghai reported the highest number - 55 passengers and 13 crew. All are at a hotel in Pudong district, Xu Jianguang, director of the Shanghai health bureau, said yesterday.

"All of them are in fine condition, both physically and mentally," he said.

Xu said about 59 family members of passengers were also told to be under health observation at home.

In Beijing, 15 people are at Guomenlu Hotel; and 28 are under quarantine in Guangzhou, capital of Guangdong.

In Hong Kong, the risk of the flu spreading exists as authorities have not located some 50 guests at the hotel where the Mexican briefly stayed, as well as the taxi driver who took the patient to hospital.

Some 300 guests and staff of Metropark Hotel in Wan Chai, where the patient stayed, are quarantined.

One of the guests, surnamed Yang, said he was frustrated.

"My flight back to Beijing will depart tomorrow. My permit will expire," he said.

Hong Kong's Home Affairs Department director Pamela Tan said the government will help arrange flight tickets for the guests, and pay for the cost.

In Beijing, Health Ministry spokesman Mao Qun'an admitted yesterday that the first confirmed case in Hong Kong has "greatly increased the possibility of the epidemic spreading to the mainland".

"Our precautionary measures face a real test," he said.

Starting yesterday, the ministry began a daily surveillance report on the epidemic. The "zero report" mechanism, which was introduced during the SARS epidemic in 2003, requires local health authorities to submit a daily report on the disease even if no case is identified.

Meanwhile, anyone coming from places where the flu was reported should have their temperature checked twice, said the General Administration of Quality Supervision, Inspection and Quarantine (AQSIQ).

People entering the country are required to fill in personal health statement cards to strengthen control efforts, it said.

According to the World Health Organization, 17 countries and regions reported 787 cases by yesterday afternoon.

The virus has killed 19 in Mexico and one in the United States, the WHO said.

China has developed an effective method for quick diagnosis which takes less than 12 hours after samples are taken, said Zhang Yanping, a researcher with the Chinese Center for Disease Control and Prevention (China CDC).

The kit will be sent to 84 labs across the country, he said.

China is also awaiting samples of the virus from the WHO to produce its own vaccine, Zhang said, adding that it could take at least three to four months for production.

Even with all these measures, experts said, it is not possible to block the spread of the disease completely.

"Infected people don't show any symptom during the latent period. It's extremely hard to find them," said Wang Jian, another researcher with China CDC.