The population-based incidence of admission to hospital with laboratory-confirmed pandemic (H1N1) influenza was low in the first five months of the pandemic in Canada. This experience is consistent with that of other countries.9–12
As in Canada, the highest rates of hospital admission in the United States, Australia and the United Kingdom were among children less than five years old.13–15
Cumulative rates of death during the study period were also low: in Canada 0.21 per 100 000 population died, as compared with 0.19 confirmed deaths per 100 000 in the United States.11
Rates of death in New Zealand, Australia, Chile, Paraguay, Argentina and South Africa, which experienced the first wave of the pandemic during their usual influenza season, ranged from 0.19 to 1.4 per 100 000 over the same period.12
In Canada, the first wave of the pandemic had a geographically and chronologically heterogeneous distribution. However, the risk of a severe outcome among patients admitted to hospital remained relatively constant during the study period and is consistent with other reports that the period and point prevalence of admission to ICU among inpatients ranged from 12% to 20%.16,17
Requirements for mechanical ventilation may have been underreported through public health surveillance, since a Canadian study reported that 81% of patients admitted to ICU required mechanical ventilation.18
The demand for ventilators and ICU beds during the second wave may be mitigated by the availability of a vaccine and by earlier and broader use of antiviral agents.
The risk of a severe outcome was greatest among the inpatients who had one or more underlying medical conditions and those who were 20 years of age or older. Those aged 65 years and older were at greatest risk of death. Children less than 10 years old, pregnant women and Aboriginal people were not at increased risk of a severe outcome, despite the relatively high population-based incidence of these outcomes. This paradoxical finding may be related to pediatric, pregnant and Aboriginal patients being admitted with milder disease and thus a lower chance of a severe outcome, or to the fact that population-based rates are crude measures that combine both probability of exposure and probability of virulent infection. Children are considered key transmitters of influenza and responsible for seeding households with the infection, so their likelihood of exposure may have been higher in the first wave.19
The risk of death from seasonal influenza is highest among people 70 years of age or older, and hospital admissions because of seasonal influenza are highest among children less than two years old and adults over 65 years.3,20
In the case of pandemic (H1N1) influenza, hospital admissions and severe outcomes occurred in all age groups and in a much younger Canadian population than seasonal influenza typically does. The age distribution of patients admitted to hospital in Canada was similar to that in the United States, where the median age of patients admitted to hospital was 20 years and the median age of death was 37 years.4
In our study, children had the highest absolute risk of severe and nonsevere outcomes, perhaps because of the tendency for influenza attack rates to be highest among preschool and school-age children.19
In terms of the timing of the pandemic, the peak of the first wave coincided with a period when Canadian students were in school. Resolution of the first wave coincided with the beginning of the summer break in July. Although children congregate in other places in the summer, school breaks have been reported to slow or delay the impact of seasonal influenza and may have played a role in interrupting transmission.21
Patients 65 years of age and older experienced the lowest incidence of hospital admission without a severe outcome but the highest population-based rate and relative risk of death among those admitted to hospital. This finding suggests the possibility of varying levels of immunity in this age group or a reduced penetration of the virus into the elderly community in the first wave.
Our analysis confirmed that the presence of one or more underlying medical conditions known to predispose to complications of influenza contributed to an elevated risk of a severe outcome among patients with pandemic (H1N1) influenza. Further research is needed to stratify risk based on the severity of the underlying conditions to further focus recommendations for prevention and treatment. In our study, underlying lung disease was common in all three patient groups (those with a non-severe outcome, those admitted to ICU and those who died), a factor that may have contributed to the lack of elevated risk of admission to ICU among patients with lung disease. Given its prevalence in our study cohort, lung disease should continue to be viewed as an important risk factor for hospital admission among patients with pandemic (H1N1) influenza. Among patients without underlying medical conditions, the risk of severe outcomes appeared to be focused on those 30–49 years of age and patients 65 years and older. A similar demographic shift has been noted in other pandemics.22,23
We cannot rule out some age-related correlation with other conditions we did not routinely survey (e.g., obesity).
We found that the risk of a severe outcome was not greater among Aboriginal patients than among non-Aboriginal patients. However, high population-based rates of hospital admission because of pandemic (H1N1) influenza have been reported among Canadian Aboriginal people, a pattern similar to that seen in New Zealand, where Maori and Pacific peoples had higher rates of hospital admission (43.0 and 94.2 per 100 000 respectively) than those of European descent (14.1 per 100 000).16
Similarly, indigenous Australians were 10 times as likely to be admitted to hospital as nonindigenous Australians.24
Demographic and clinical factors such as younger age distribution and higher prevalence of underlying conditions in Aboriginal communities may be at play; however, additional contributing factors, including the role of socio-economic and geographic factors, and possibly genetic susceptibility, need to be explored.25
Over the study period, 78 pregnant women were admitted to hospital with pandemic (H1N1) influenza. Typically, 300 pregnant women in Canada are admitted to hospital because of influenza each year, which corresponds to the rate observed among men and women 65–69 years of age.26
Clinicians may therefore expect the rate of hospital admission because of pandemic (H1N1) influenza to be higher among pregnant women than in the general population.
A delay of one day in the median time between the onset of symptoms and admission to hospital was associated with an increased risk of death. However, we did not have the information to determine whether this finding was related to a delay in treatment. A report from the United States showed that, in a multivariable model including age, vaccination status, time to hospital admission and time to antiviral treatment, the only factor associated with positive outcomes was the receipt of antiviral treatment within 48 hours after onset of symptoms.27
We did not include probable or suspect cases of pandemic (H1N1) influenza in our analysis. In addition, our inclusion of cases from two provinces that were missing information on underlying medical conditions may have underestimated the role of such conditions in causing severe outcomes. Finally, data on Aboriginal status was not reported by two provinces.
The population-based incidence of admission to hospital with laboratory-confirmed pandemic (H1N1) influenza was low in the first five months of the pandemic in Canada. The risk of a severe outcome was associated with the presence of one or more underlying medical conditions, age of 20 years or more and a delay in hospital admission. The ability to gather detailed, case-based information rapidly and in a relatively uniform manner across Canada reflects an important partnership between provincial, territorial and federal public health authorities. As the pandemic evolves, continued investigation of risk factors for severe outcomes is needed to provide timely evidence to inform the development and updating of clinical and public health guidelines.