The World Health Organization on Monday published a new plan on how to alert the world to possible flu pandemics, following harsh criticism of its handling of the H1N1 swine flu pandemic in 2009.

The UN's health agency said it had simplified its alert system and redefined what constitutes a pandemic to put more emphasis on the risk it posed instead of just focusing on its spread.

"The key point of the new guidance reflecting the lessons learnt (was to make it) very much risk-based," WHO expert David Harper told reporters in Geneva Monday.

The changes came after the agency faced a barrage of criticism for how it handled the first flu pandemic of the 21st century.

The WHO announced H1N1 swine flu had reached pandemic proportions on June 11, 2009, first sparking panic-buying of vaccines and then anger when it turned out the virus was not nearly as dangerous as first thought.

Swine flu killed more than 18,449 people and affected some 214 countries and territories, but the world had been bracing for far worse, and governments stuck with millions of unused vaccine doses were especially upset.

In March 2011, a WHO evaluation committee called on the organisation to simplify its description of a pandemic to make it more precise and consistent and to assess the risks and severity of a pandemic.

It also called for the agency to improve both routine and emergency communications to the public.

In the previous system, a pandemic was declared when a virus caused community level outbreaks in at least two different WHO-defined regions, and in at least two countries in one WHO region.

The definition of a pandemic has now been simplified to a "period of global spread of human influenza caused by a new subtype," WHO said.

The new system had been simplified and used four phases -- interpandemic, alert, pandemic and transition -- instead of the previous seven to describe the spread of a new influenza subtype, taking account of the diseases it causes around the world, WHO said.

According to the new rules, for instance, WHO currently considers the world is at the "alert" level when it comes to both the H5N1 and H7N1 bird flus, compared to level three in the old system, Harper said.

If there is concern that a new pandemic has broken out, the WHO secretariat will urgently convene a group of experts to counsel the head of the organisation, who will in turn decide whether to put the world on the pandemic phase.

The new system also aims to encourage countries to develop their own risk assessments and plans to address a potential pandemic, including closing schools and sports stadiums, Harper said.