GENEVA — Health experts are looking very closely at the spread of swine flu among people in Spain, Britain and Japan, a WHO official said yesterday as Japan reported an explosion over the weekend to more than 120 new cases, mostly among teenagers.

The swine flu epidemic is already expected to dominate the World Health Organization's annual meeting, a five-day event that begins today in Geneva and involves health officials from the agency's 193 member states.

WHO Director-General Dr. Margaret Chan will reveal experts' recommendations on the production of a swine flu vaccine sometime at the meeting. Pharmaceutical companies are ready to begin production, but many decisions have to be made first — such as how much vaccine to make, how it should be distributed and who should get it.

As of yesterday, the swine flu virus — which WHO calls the A (H1N1) virus — has sickened at least 8,480 people in 40 countries, killing 76 of them, mostly in Mexico.

Japan's Health Ministry confirmed dozens of new cases of swine flu in waves of announcements yesterday, prompting the government to shut down schools and cancel public events, including Kobe's annual festival. By today, Japan's tally rose from five confirmed cases to at least 121 — many of them high school students in the western prefectures of Hyogo and Osaka who had not traveled overseas.

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