WHO chief Margaret Chan on Thursday listed criteria that might prompt her to declare a flu pandemic, including the spread of swine flu in the southern hemisphere, an official told AFP.
Chan said she would not hesitate to move to the top phase six of the pandemic alert system "if it was appropriate." But during a committee meeting of the World Health Organisation's annual assembly she also gave member states several key reasons why there was no reason to do so at the moment with the swine flu outbreak.
"One of the things we're not seeing is the same spread in the southern hemisphere that we've seen in the first three countries," the official explained.
The southern hemisphere is currently entering its standard influenza season, and health experts fear that the new A(H1N1) virus could mix with seasonal strains.
The WHO's Director General also pointed to the relative lack of severity of the virus, which so far has only matched seasonal flu in terms of its symptoms according to experts.
On April 29, Chan raised the global flu alert level to phase five - marking an "imminent" pandemic -- following the outbreak of the new influenza A(H1N1) in Mexico and the United States .
The spread of swine flu to 41 countries, including the swift infection of 281 people in Japan in recent days, has fuelled speculation that the alert level could be ramped up to phase six.
WHO officials have also underlined in recent weeks that they were looking for sustained transmission in the wider community outside North America, not only cases that involve direct transmission from travellers or among groups of students.
During the committee meeting, several countries -- including Britain, one of the countries in Europe most affected by swine flu --- reiterated their concerns about the "mechanistic" nature of the flu alert system and the need for a flexible approach, the official said.