HONG KONG - The head of the World Health Organization said Monday the swine flu virus had apparently not yet mutated into a more serious disease and that the development of vaccines was proceeding on track.

The vaccines for (A)H1N1 influenza produced so far have been very effective, WHO Director-General Margaret Chan said at the opening ceremony for the organisation's annual Western Pacific meeting in Hong Kong.

"The virus can mutate any time. But from April to now, we can see from the data given to us by laboratories worldwide that the virus is still very similar (to the previous state)," Chan told reporters.

Chan said the biggest challenge in combating the pandemic would be ensuring enough vaccine got to the world's poorest countries.

Ideally, three billion doses could be produced worldwide annually, she told the meeting, noting that China had already begun to vaccinate people.

"Results of early clinical trials suggest that a single dose of pandemic vaccine will be sufficient. If confirmed, this finding will literally double the amount of vaccine available," Chan said.

"Here's the big question: Will this result in more equitable distribution of vaccines? Let me assure you: I am pursuing this opportunity from several angles."

Chan warned that the death of pregnant women in the developing world during a pandemic would be "especially tragic" because the number could be much higher than elsewhere.

Her comment came after the WHO's warning last week that the annual production of swine flu vaccines is expected to fall well short of the 4.9 billion doses that it had earlier forecast.

And on Sunday, WHO regional director Shin Young-soo told a press conference that his group and the United Nations are working on raising a billion dollars to help buy vaccines for countries that need help.

In the Western Pacific there are about a million people living in poor conditions without access to healthcare, he said.

Chan said Hong Kong could relax its measures against a swine flu outbreak "step-by-step", advising it to focus long-term resources on saving patients and reducing the number of serious cases.

Chan said that only high-risk patients such as the elderly, the obese and those with underlying illnesses would be severely affected by the disease.

Swine flu in Hong Kong has raised fears of a repeat of the 2003 Severe Acute Respiratory Syndrome (SARS) outbreak, when almost 300 people died and concern about the mysterious disease turned the metropolis into a virtual ghost town.

Chan managed Hong Kong's response to avian influenza and SARS during her nine-year stint as Hong Kong's director of health.

Top of the agenda for the WHO's meeting this week will be how to combat the swine flu pandemic in developing nations.

While the Americas still has the highest death toll from the virus, cases are expected to increase in the region as the northern hemisphere enters winter.

There are fears that poorer countries will not get enough vaccines, despite a pledge last week by the United States and eight other nations to make 10 percent of their swine flu vaccine supply available to others in need.

Developing countries are not only unable to produce the vaccine for the A(H1N1) flu virus but their people are more vulnerable to infection because of poverty, crowded living conditions and lack of healthcare, according to the WHO.