A month after the world was alerted to a potential flu pandemic, the WHO was caught Thursday between the spread of the new swine flu virus to 41 countries and doubts fostered by its mild symptoms.
Some 10,000 cases and 80 deaths have been recorded since the outbreak of A(H1N1) influenza emerged in Mexico and the United States, and the world remains at flu alert level five, signalling an "imminent pandemic".
But World Health Organisation Director General Margaret Chan is hesitating about declaring a fully-fledged pandemic by moving to phase six, even though travellers have carried it to other continents.
The top level would indicate sustained community transmission in a second region outside the Americas.
On Thursday, Japanese Prime Minister Taro Aso appealed for calm as a total of 281 swine flu cases were reported in the country, including the first in greater Tokyo, the world's largest urban area.
Antoine Flahaut, an epidemiologist and head of the School of Public Health (EHESP), told AFP that the technical elements were in place to move into the pandemic phase.
"But the WHO senses that recommendations which go with that are not adapted to the situation," he explained, pointing to air travel restrictions or advice to wear surgical masks.
"Invoking phase six would be disproportionate with the current situation."
The doubts have grown because of the relatively mild symptoms of swine flu, which experts acknowledge is no worse than seasonal influenza for now.
Many of the deaths have occurred among those who were suffering from other ailments, a common pattern for ordinary strains of flu.
When the WHO's annual assembly opened on Monday, British Health Secretary Alan Johnson voiced doubts about phase six that had been growing behind the scenes after countries rushed to contain new cases of swine flu.
"I think you, as you and others have said, need more time, we need more time to study this," Johnson told Chan, prompting nods of approval from other health officials afterwards, including China, Japan and New Zealand.
"She has taken that on board," WHO spokesman Thomas Abraham said late Thursday, underlining that most of the cases in Japan were largely confined to students or their close entourage.
Chan acknowledged this week that the WHO's pandemic response plan, introduced three years ago, was largely designed around the more deadly and virulent, but less transmissible, H5N1 bird flu virus.
"This scale was based on geographical distribution, but the public belief is that pandemic means seriousness," said Sylvie Briand, acting director of the WHO's Global Influenza Program.