New cases of the swine flu virus soared in Japan and Canada Thursday but the World Health Organisation held back from declaring a pandemic due to doubts fuelled by the mildness of symptoms.
More than 11,000 cases and 85 deaths have been recorded since the outbreak of A(H1N1) influenza emerged in Mexico and the United States a month ago, and the world remains at flu alert level five, signalling an "imminent pandemic."
WHO Director General Margaret Chan has stopped short of declaring an all-out pandemic by moving to phase six, even though travellers have carried it to other continents and a total of 41 countries.
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 the number of cases reached 281, including the first in greater Tokyo, the world's largest urban area.
In Canada, authorities said the number of swine flu cases had soared by more than 200 since the end of last week.
"Of the 719 cases, 13 have required hospitalisation, which supports what we have seen so far in Canada, that the severity of the symptoms is more typical of seasonal flu," said Canada's Chief Public Health Officer David Butler-Jones.
"However, even with a mild flu, we need to remain vigilant in preventing illness and watching for changes in the virus."
Australia on Thursday confirmed two new cases taking the total number to seven. A primary school was closed for a week in Melbourne after two brothers were diagnosed with swine flu.
"I think it's inevitable that we will have community transmission, we've seen in other countries that inevitably there is community transmission," said Rosemary Lester, chief health officer of Victoria state whose capital is Melbourne.
Antoine Flahaut, an epidemiologist and head of the School of Public Health (EHESP) in France, 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."
Doubts on declaring a full-scale pandemic have grown due to the relatively mild symptoms of swine flu, which experts say 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.
During a meeting at the WHO's annual assembly Thursday, Chan listed the issue of swine flu's severity among other criteria that were prompting caution.
"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," a WHO official added.
When the 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.
Those calls for caution and a more flexible system prompted nods of approval from other countries afterwards, including China, Japan and New Zealand.
"She has taken that on board," WHO spokesman Thomas Abraham said.
Chan acknowledged this week that the WHO's pandemic response plan, introduced nearly four years ago, was largely designed around the more deadly and virulent, but less transmissible, H5N1 bird flu virus.
In studies released by the New England Journal of Medicine, scientists pointed out the similarities between the new A(H1N1) virus and ones behind pandemics that marked the 20th century.
Those pandemics in 1918-1919 -- which killed an estimated 50 million people -- in 1957-1963 and 1968-1970 started off as mild but went through waves that became more lethal at their peak, often the second season, and had different impacts in different regions.
They also affected young people -- a feature underlined by the WHO in the current outbreak -- and were highly transmissible, the researchers from the US National Institutes of Health and George Washington University said.
Abraham said it was still not clear if young people were more affected by swine flu because of the nature of the virus, or because it was more likely to be passed around among students in the close confines of schools.
