Passengers wearing protective masks take a subway in Mexico City May 6, 2009. The Mexican capital faced a five-day shutdown to try to contain the deadly H1N1 flu. (Jorge Dan/Reuters)

More than two-thirds of people with severe pneumonia in the first days of Mexico's swine flu outbreak were between the ages of five and 59, researchers reported Wednesday.

In an early online issue of the New England Journal of Medicine, researchers from Mexico and the U.S. looked at 2,155 cases of severe swine flu among people confirmed with H1N1 virus infection.

Of these, 71 per cent of the cases were neither very young nor old, compared with an average of 32 per cent for seasonal flu.

And 87 per cent of the deaths recorded between the study period of March 24 and April 29 were patients between the ages of 5 and 59, compared with an average rate of 17 per cent for seasonal flu.

Rationing vaccine

During the early phase of the 2009 influenza pandemic, there was a sudden increase in the rate of severe pneumonia and a shift towards a younger age distribution — a change reminiscent of past pandemics, epidemiology researcher Dr. Mark Miller of the U.S. National Institutes of Health in Bethseda, Md., and his colleagues said.

"If resources or vaccine supplies are limited, these findings suggest a rationale for focusing prevention efforts on younger populations."

There is no clear explanation for why the new virus has a higher and more severe attack rate among younger people.

Part of the speculation is that older people who survived previous pandemics where H1N1 circulated may have developed some immunity. Another possibility is that the immune system of younger people may go into a sort of lethal overdrive to fight off the virus.

Canadian public health officials are debating what segments of the population, such as younger people or those at risk of serious disease, including pregnant women, should receive priority for a potential pandemic vaccine in the fall.

Most Canadian cases under age 20

In Canada, the number of reported cases of H1N1 has grown by the hundreds in the last couple of weeks to more than 7,700, but fewer than 500 have needed treatment in hospital.

"The majority of cases we're seeing are younger, the average age is less than 20," said Canada's Chief Public Health Officer, Dr. David Butler-Jones. "That appears to be where much of the spread is taking place, so whatever the final recommendation, this will not be a typical flu season."

The federal government is ordering enough pandemic vaccine to offer it to all Canadians who wish to roll up their sleeve for it in late fall. Annual seasonal flu shots will also be offered.

A letter appearing in the same issue of the journal by Canadian researchers showed the international destinations of air travelers leaving Mexico, including Canadian cities, were strongly associated with confirmed importations of the virus worldwide.

"We know that infectious diseases don't respect national boundaries, but if there is one thing they have to respect as they're spreading around the globe, it's the architecture of the airline transportation system," said physician Dr. Kamran Khan of St. Michael's Hospital in Toronto, who developed the system to accurately predict the spread of swine flu.

"This is their mechanism for spreading around the globe."

In a 122-page report to the Public Health Agency of Canada, the researchers found 13 countries produce 80 per cent of air traffic to Canada.

Four developing countries — China, Mexico, India and the Philippines — and nine industrialized countries — the U.S., the U.K., France, Germany, Italy, the Netherlands, Japan, South Korea and Taiwan — are the source of 80 per cent of Canada's international air traffic.

It is these countries that are key places where future infectious disease threats may start or pass through en route to Canada, Khan said.

With files from the Canadian Press and the Associated Press