H1N1 pandemic influenza remains a cause for concern because of its unpredictable nature, even though it has killed fewer than 5,000 people so far this year, the World Health Organisation said on Friday.

A statement from the United Nations health agency said that more than 4,735 deaths attributable to H1N1, known asswine flu, had been reported, and that influenza activity in the northern hemisphere was much higher than usual.

But WHO spokesman Gregory Hartl said it was too soon to draw any conclusions from the death toll as experts needed to monitor a full year of the disease, which the WHO declared a pandemic in June after the strain was first detected in April.

Although the death rate might not be enormous at the moment we do have to continue to be prepared for developments as we go through the winter in the northern hemisphere, Hartl said.

In particular, health experts need to observe the behavior of the virus during the traditional January-February peak of the influenza season in the northern hemisphere, he told a briefing.

Most people who catch the H1N1 virus suffer mild symptoms.

But in contrast to seasonal flu strains which can be serious for elderly people, H1N1 can turn dangerous for some people with existing health conditions or otherwise healthy young adults.

There is a small subset of cases that do and can progress quite rapidly to severe disease and this is sometimes in the space of less than 24 hours and it then becomes a big, big challenge to save the people, Hartl said.

This disease continues to cause concern because it doesn't act exactly like seasonal influenza and because it doesn't affect the same groups who are affected by seasonal influenza.

Medicine