Canadians who died of H1N1 were, on average, 30 years younger than those who typically die of seasonal flu, newly-released data show.
Infection with the pandemic strain of influenza also resulted in markedly more cases that required treatment in intensive-care units and resulted in patients requiring mechanical ventilation to breath. But, paradoxically, there were fewer deaths than a run-of-the-mill flu, according to an analysis by the Canadian Institute for Health Information.
"We've had good data collection throughout the pandemic and this helps put it into context," said Kathleen Morris, head of emerging issues at CIHI, said in an interview.
"We hope this information will serve as a springboard for understanding influenza - pandemic and seasonal - and the impact it has," she said.
The study consists of a comparison of hospitalizations and deaths related to H1N1 in 2009-10 and a seasonal flu in 2007-08.
Since H1N1 surfaced in March of last year, there have been 426 deaths attributed to the virus in Canada. In 2007-08, there were almost 8,000 deaths due to influenza and pneumonia. (It should be noted, however, that those numbers are not directly comparable.)
The CIHI report focuses principally on hospitalization. It shows that, between April 2009 and January 2010, there were 8,507 Canadians hospitalized for treatment of pandemic influenza.
A couple of years earlier, there were significantly fewer patients hospitalized for treatment of seasonal flu - 2,614 in total.
In other words, while fewer people seem to have fallen ill with H1N1, a significant number of those who fell sick were seriously affected.
The most striking distinction between pandemic and seasonal flu, however, is one that became obvious early on - that the former hit young people disproportionately hard.
The median age of those who died of H1N1 is 53. For the seasonal flu, it is 83.
For those hospitalized, the gap is even more striking: The median age of those hospitalized with H1N1 was 28, compared to 71 for those with seasonal flu.
While H1N1 did not hit near as hard as some public health officials had feared, it strained hospitals because of the number of people who needed specialized care in intensive care units. One in six of those hospitalized due to H1N1 ended up in intensive care, and one in 10 ended up on a ventilator.
By comparison, one in seven patients hospitalized with seasonal flu required treatment and intensive care, and one in 30 needed mechanical ventilation.
"This kind of information can really help when hospitals review their pandemic plans," Ms. Morris said.
The new report is a statistical analysis which does not offer reasons for the age-related differences. The most common explanation is that, while the flu typically preys on the frail elderly, it appears that people born prior to 1957 had some immunity to H1N1. (Between 1918 and 1957, the predominant strains of circulating influenza were from the H1N1 family.)
While several smaller studies have concluded that H1N1 hit women particularly hard, the CIHI report shows that women and men have been hospitalized and died in about equal numbers.
In related news, the World Health Organization said yesterday that it planned to convene a panel of flu experts to formally determine if the pandemic has passed its peak.
André Picard