TORONTO - H1N1 influenza landed a higher proportion of Canadians in ICUs and on ventilators, and at a much younger age on average, than seasonal flu does in a typical year, says a report assessing the comparative impact of the pandemic virus.
The study by the Canadian Institute for Health Information compared hospitalization statistics for H1N1 patients between April and December to data for patients with regular flu during the 2007-2008 season.
"Initially there were clinical perceptions of the patients: they seemed younger, they seemed sicker," co-author Kathleen Morris said of the H1N1 pandemic that began last April.
"What this does is take a look at all of the H1N1 patients that were hospitalized in Canada and compare them to a full hospitalization (seasonal) flu year, so right across the country," Morris, head of emerging issues at CIHI, said Thursday from Ottawa.
"So it gives us a clearer sense of A, whether those findings are confirmed by the data, and B, what magnitude is the difference."
She said researchers were surprised at how big some of the differences were.
"People admitted to hospital with H1N1 were younger than what we'd come to expect in a typical flu year, and those who died were younger, too," said Morris.
The median age for patients hospitalized with H1N1 was 28, compared to 71 for people admitted for seasonal influenza or flu-related pneumonia. Among those sent to the ICU, the median age of H1N1 patients was the mid-40s, while seasonal flu patients in a typical year had a median age of 68.
There was a staggering difference in the median ages of those who died: the mid-50s for H1N1 and the mid-80s for those from complications of seasonal flu, the most common of which is pneumonia.
"So that's something about the patient that seems to be a bit different," Morris said. "There's also something about the kinds of care they needed in hospital that seemed to be different."
The researchers found that a higher proportion of patients in hospital for H1N1 were admitted to an intensive care unit -- almost one in six compared with about one in 10 of those with seasonal flu or flu-related pneumonia.
And when it came to the proportion of patients who needed a ventilator to assist breathing, H1N1 patients were more than twice as likely to require the machines.
"We also know that H1N1 appears to have had a greater impact on some subgroups of the population -- like pregnant women -- than what we'd expected in a typical year," Morris said.
The study found the proportion of pregnant women hospitalized with H1N1 was higher than what would be expected in a typical flu year -- 21 per cent versus 13 per cent.
Four of the pregnant women with H1N1 died, she said. "And when we looked in all of 2007-8, there were no deaths among women hospitalized with flu or pneumonia."
Commenting on the study, Dr. Michael Gardam said that while there are no big surprises in the report, it provides another perspective using a different set of data.
"Most people with it did have mild disease, but it really points out that actually there were a substantial number of people who were quite sick who ended up ventilated," said Gardam, head of infection prevention and control for Toronto's University Health Network.
"It just reinforces how lucky we were, that had this virus had a little bit more of the right stuff, we could have been in really big trouble."
Gardam believes we've seen the end of the H1N1 influenza pandemic because such a high proportion of Canadians has either been infected with the virus or been vaccinated against it.
"It becomes very difficult for the virus to take off... You're still going to get cases, but you're not going to get those big peaks like we had in the first and second wave because there simply is not enough fuel."
Morris said that while the CIHI study answers a lot of questions about what happened with H1N1, more research is needed to answer the why questions, as in "Why did it behave the way it did?"
"We hope that it's a good springboard for more study, because there's still a lot that's not known about H1N1."
