The chief of the World Health Organization says she is not raising the world swine flu alert level just yet.

Several countries including Britain, Japan and China had urged the U.N. health agency to change how it decides to raise the alert level.

WHO chief Dr. Margaret Chan says the swine flu epidemic is in "a grace period" with the WHO alert remaining at phase 5 out of a possible six for the last month. She told the WHO annual assembly on Monday that no one can say how long this period will last.

Chan says the danger now is that the swine flu virus could mix with other flu strains and become more dangerous.

Swine flu and the possibility of a vaccine topped the agenda Monday as the World Health Organization opened its annual meeting amid concern that the virus continues to spread — and kill — around the globe.

WHO said health experts are examining newly reported cases in Spain, Britain and Japan, where more than 120 people have been infected, prompting school closures and cancellations of public events.

Meanwhile, officials from Britain, Japan and China believe the WHO's current system for alerting the world of a potential pandemic focuses too much on how widespread the disease has become without regard to its severity.

Some member nations are anxious to avoid having the agency declare a swine flu pandemic, because the ramifications of that scientific decision could be very costly and politically charged.

"We need to give you and your team more flexibility as to whether we move to phase 6," British Health Secretary Alan Johnson told the WHO Monday.

Japan also called for changes in WHO's system, which would move to pandemic if the virus starts to be transmitted among people outside schools, hospitals and other institutions where viruses typically pass quickly.

"It's certainly something we will look at very closely," Dr. Keiji Fukuda, WHO's flu chief, said of the proposal.

So far, the United States was noncommittal on the issue. U.S. Health and Human Services Secretary Kathleen Sebelius told The Associated Press she wanted more information on the proposal before taking a position, but that she was impressed how many countries supported it.

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The five-day meeting in Geneva, which involves health officials from the agency's 193 member states, will focus on fighting the swine flu outbreak and efforts to produce a vaccine.

Chan will give experts' recommendations on which companies should produce a vaccine, how much they should make and how it could best be distributed.

The issue of producing a vaccine is sensitive, particularly for southern hemisphere countries where the annual flu season is about to begin. Seasonal flu can claim as many as 500,000 lives a year globally. But to have enough vaccine to confront a pandemic from a new strain such as swine flu, companies would switch production from vaccine production for seasonal flu.

WHO estimates up to 2 billion doses of swine flu vaccine could be produced yearly, though the first batches would not be available for four to six months.

As of Sunday, the swine flu virus — which WHO calls the A (H1N1) virus — has sickened at least 8,480 people in 40 countries, killing 75 of them, mostly in Mexico.

Chile was the latest country to announce its first case of swine flu on Sunday.

Japan's Health Ministry confirmed dozens of new cases Sunday, prompting the government to close schools and cancel events such as Kobe's annual festival. By Monday, Japan's tally rose from five confirmed cases to more than 120.

Most of the new cases involved high school students in the western prefectures of Hyogo and Osaka who had not traveled overseas. Health officials said they were recovering in local hospitals or at home.

WHO spokesman Gregory Hartl said in-country transmission rates were a key factor in whether the global body decides to increase its pandemic alert level. Right now, the world is at phase 5 — out of a possible 6 — meaning a global outbreak is "imminent."

"We already know about the UK and Spain, that they have a relatively high number of cases compared to other European countries. So by simple virtue of the fact that they have more cases they need to be kept an eye on," Hartl said in an interview Sunday with AP Television News.

"There seems to have been activity in the last few days in Japan so we need to watch that, too," he said.

Spain and Britain have had the highest numbers of cases in Europe, reporting 103 and 101 cases, respectively. Britain on Sunday announced 14 new cases — 11 of them transmitted within the country.

A pandemic could be triggered if the virus starts to be transmitted from person to person on a large scale outside the Americas, WHO experts have said. But it would have to jump among people outside schools, hospitals and other institutions that typically pass on such viruses quickly.

"We don't want to prejudge anything, but certainly this is something we are watching with interest," Hartl said of the weekend developments in Japan.

U.N. Secretary-General Ban Ki-moon will visit WHO on Tuesday to meet with senior representatives from the vaccine industry, but the U.N. declined to name the companies.

WHO's health assembly will run through May 22, five days shorter than initially planned because health ministries are busy fighting the swine flu outbreak.

Taiwan received an observer seat on the World Health Assembly and is taking part for the first time in 38 years.