A third of the world's population could be infected by swine flu in the next year, a top United Nations health official said on Thursday, urging Asian governments to stay alert for a potentially wider pandemic.
Keiji Fukuda, acting assistant director-general for the World Health Organisation, also said it was "quite likely" it would declare a pandemic in the near future but a final decision had not been made.
"This is a disease that could potentially infect a third or more of the world's population in the next several months, in the next year," he told Asian health officials.
He added that "even if the illnesses appear relatively mild on a global level, the global population level adds up to enormous numbers".
Health ministers from Japan, China, South Korea and the 10-member Association of South East Asian Nations will attend the Bangkok meeting on Friday to discuss efforts to coordinate their fight against the H1N1 virus.
Thai soldiers and police imposed tight security around the meeting venue on Thursday to prevent any recurrence of the violence that forced an Asian leaders' summit to be cancelled in April.
The virus, which has killed 44 people in Mexico and the United States and spread through Europe, has not appeared widely in Asia so far. There are five confirmed cases in New Zealand, three in South Korea and one in Hong Kong.
After bouts with SARS and bird flu in recent years, Asian health officials said they were better prepared to handle a pandemic, with stronger surveillance systems, laboratories and stockpiling of antiviral drugs.
But David Nabarro, the UN influenza coordinator, worried governments might get complacent because many people in harder-hit countries had experienced only mild symptoms from the flu and recovered without medicine.
He said the most serious flu pandemic of modern times, which killed 40 million people in 1918-19, started with a milder early wave of infections.
Mr Fukuda said there was no decision yet on whether to revise the WHO's pandemic alert, now at 5. He said it could drop to 4 or rise to the top of its 6-point scale, which would activate emergency response plans to fight the virus.
China lifts quarantine
China on Thursday lifted a seven-day quarantine for all but one of the passengers who arrived in Shanghai on the same flight as a Mexican found to be infected with H1N1 flu, and again denied discriminating against foreigners.
One of the 128 passengers showed flu-like symptoms and was still undergoing medical tests, said Xinhua, citing a Health Ministry report.
The passengers took a 1 May flight from Mexico City, which stopped in Shanghai on its way to Hong Kong.