OTTAWA - Canadian health officials expressed alarm Monday that the swine flu is affecting younger people hardest.

Preliminary data shows that the A(H1N1) virus has mostly infected people under the age of 20 in Canada, and relatively few people over 65 have contracted it.

During seasonal influenza outbreaks, it is usually the elderly who account for 25 percent of infections, and most deaths.

"So this is quite different," Chief Public Health Officer David Butler-Jones told a press conference.

"The vast majority of the cases continue to be mild, at least mild for influenza, but we are seeing some pockets of more severe illness," he said.

"Although we do expect some cases to be severe in any influenza outbreak, especially when there is underlying factors such as chronic lung disease or diabetes, there are some cases (now) where the individual was previously healthy before catching this virus and rapidly has progressed to severe illness and required a ventilator," he said.

"We're obviously concerned by these severe cases," he added.

Butler-Jones said health authorities are closely monitoring the severe cases to try to understand why some people are suffering worse symptoms than others.

He suggested "the virus itself may be changing and becoming more virulent," or there may be "immunity differences" or other factors that could account for the differences.

But "we don't have an answer yet," he said. "We're still speculating, (and) there is a whole range of possibilities that are being looked at."

Canada has confirmed 7,983 cases of swine flu nationwide, the third-largest number in the world, including 636 requiring hospitalization, and 25 deaths.

An A(H1N1) flu vaccine is being developed and is to be available by late October, officials said.

Meanwhile, Health Minister Leona Aglukkaq is to travel to the United States and Mexico in the coming days to discuss "lessons learned" so far in this pandemic, she said.