Kids with asthma are at much higher risk of developing severe illness from H1N1 flu than they are with seasonal flu, a new Canadian study has found.

The study, led by researchers from The Hospital for Sick Children in Toronto, looked at the medical charts of 58 children who had to be admitted to SickKids with a severe H1N1 infection, between May 8 and July 22, 2009.

They then compared them to 200 children admitted with seasonal influenza, between 2004 and 2009. Every year, the hospital admits about 40 children with seasonal influenza.

Swine flu appeared to be much more of a risk for kids with asthma than regular flu: 22 per cent of kids admitted with H1N1 had asthma, compared with six per cent of those admitted with seasonal influenza.

In fact, almost half of all admissions to the intensive care unit at SickKids Hospital for H1N1 influenza were children with asthma.

It didn't matter how severe a case of asthma the children had; even those with mild asthma were at risk, found the study, which is published online in CMAJ, the Canadian Medical Association Journal.

The researchers also noticed that kids with swine flu tended to be older than kids usually admitted for seasonal flu, with significantly more over the age of five.

None of the children with H1N1 died, compared with one death over five years in those admitted for seasonal influenza. And the children tended to stay in hospital for about the same amount of time whether they had H1N1 or seasonal influenza a median time of four days.

"The most striking finding in our study was the high prevalence of asthma among children admitted with pandemic H1N1 influenza compared with those admitted in previous influenza seasons," write Dr. Upton Allen from The Hospital for Sick Children and coauthors.

"Asthma has been identified as a significant risk factor for admission with pandemic H1N1 influenza, present in 21 to 30 per cent in the larger samples."

Principal investigator Dr. Dat Tran and co-authors conclude that even children with mild asthma should be vaccinated for H1N1 and considered for antiviral therapy if they become ill.