Previously healthy patients around the age of 40 have a high risk of becoming severely ill with swine flu, a Canadian study suggests.

While most seasonal flu viruses hit children and the elderly the hardest, swine flu is atypical in that its most virulent cases appear to occur amongst otherwise healthy adults, according to a study by the Canadian Critical Care Trial Group.

"It's worrisome because it's in a new patient population we are not quite used to thinking about as being on-guard for severe illness," said Dr. Robert Fowler, a researcher with CCCTG.

Most swine flu cases are mild and many patients recover quickly. But Fowler has been tracking the small number of intense cases, and he says many of these occur in previously healthy people.

Additionally, 40-year-olds form the median age of patients requiring critical care, although the data shows that a high number of patients under five years of age need similar care.

It is believed that patients 65 and over have some immunity to H1N1, as a result of prior exposure to a flu virus which circulated in the 1950s.

On a broader note, the CCCTG researchers found that about one-third of critical H1N1 flu patients had an underlying illness when the fell ill.

The remaining two-thirds of these patients did not have a serious illness when they got sick and some were completely healthy.

While survival rates for swine flu are high, the need for aggressive treatment has some experts concerned that Canadian hospitals will be overrun if the H1N1 virus comes back in a larger wave this flu season.

"Some need aggressive support for many weeks," said Fowler, adding that such patients often require ventilators to breath.

For example, in some Australian hospitals recently, nearly a quarter of ICU beds with ventilators were taken by swine flu patients.

And the Canadian group believes that many hospitals are ill-equipped to provide such services as they lack sufficient numbers of specialized ventilators.

As a result, hospitals may need to share resources to keep their patients healthy.

According to the Public Health Agency of Canada website, more than 70 Canadians have died of swine flu since the start of the outbreak earlier this year.

The CCCTG findings will be discussed at a Winnipeg conference on H1N1 that will take place Sept. 2-3.

A Thursday news release from the Public Health Agency of Canada said the Winnipeg conference will see experts meeting to "share and discuss best practices for treating severe H1N1 infections."

Canada's chief public health officer says experts will be looking to make sure they have the best strategic tools when treating severe cases.

"Coming out of this meeting, we aim to have new guidelines on treatment and management of severe cases, as well as some guidance on issues like identifying surge capacity for hospitals during a more severe outbreak," Dr. David Butler-Jones said in a statement included in the Thursday release.

With files from CTV Medical Specialist Avis Favaro