The H1N1 virus is currently most active in parts of Europe, North Africa and South Asia, the World Health Organization says.

Swine flu has killed at least 12,799 people worldwide since it first emerged in Mexico in March 2009, WHO said Friday in its weekly pandemic update on lab-confirmed cases. At least 2,554 of the deaths were in Europe.

The number of deaths increased by 579 since the last update published nine days ago.

More than half of the deaths, at least 6,880, have been in the Americas. Overall pandemic flu activity continued to decline or remain low in both the tropical and northern temperate zones of the Americas, WHO said.

In North America, peak influenza activity occurred during early October in Mexico, followed by the United States and Canada in the middle and late part of the month.

The most active areas of pandemic influenza transmission currently are in parts of central, eastern and southeastern Europe, North Africa, and South Asia.

Poland, Serbia, Ukraine and Georgia recently reported high respiratory disease activity.

Pandemic influenza transmission is also widespread and active in northern India, Nepal, and in Sri Lanka. The same is true in East Asia, where flu spread appears to be declining overall, the WHO said.

Southern hemisphere immunity

In the southern hemisphere, H1N1 is now spreading less among people who were already exposed to it during the northern hemisphere's summer of 2009.

"In temperate regions of the southern hemisphere, sporadic cases of pandemic influenza continued to be reported without evidence of sustained community transmission," it said.

"This suggests that the level of population immunity in areas that experienced intense, high-level transmission during a winter season is high enough to prevent sustained transmission from returning during the summer when the virus is less transmissible."

Also on Friday, the European Center for Disease Prevention and Control warned governments to continue H1N1 flu vaccination programs and to watch for mutations in the virus and new strains.

"The historical pattern of human influenzas is that after pandemics, the world experiences a new mix of viruses," the centre's flu expert Angus Nicoll wrote in the Eurosurveillance scientific journal.

In the 1957-58 flu pandemics, there were increases in flu-related deaths in the new year.

It's estimated that between 250,000 and 500,000 people worldwide die from regular flu each year, including 4,000 to 8,000 in Canada, according to the WHO and the Public Health Agency of Canada.