Statistical estimates of respiratory admissions attributable to seasonal and pandemic influenza for Canada

Abstract

Background: The number of admissions to hospital for which influenza is laboratory confirmed is considered to be a substantial underestimate of the true number of admissions due to an influenza infection. During the 2009 pandemic, testing for influenza in hospitalized patients was a priority, but the ascertainment rate remains uncertain.

Methods: The discharge abstracts of persons admitted with any respiratory condition were extracted from the Canadian Discharge Abstract Database, for April 2003-March 2010. Stratified, weekly admissions were modeled as a function of viral activity, seasonality, and trend using Poisson regression models.

Results: An estimated 1 out of every 6.4 admissions attributable to seasonal influenza (2003-April 2009) were coded to J10 (influenza virus identified). During the 2009 pandemic (May-March 2010), the influenza virus was identified in 1 of 1.6 admissions (95% CI, 1.5-1.7) attributed to the pandemic strain. Compared with previous H1N1 seasons (2007/08, 2008/09), the influenza-attributed hospitalization rate for persons <65 years was approximately six times higher during the 2009 H1N1 pandemic, whereas for persons 75 years or older, the pandemic rate was approximately fivefold lower.

Conclusions: Case ascertainment was much improved during the pandemic period, with under ascertainment of admissions due to H1N1/2009 limited primarily to patients with a diagnosis of pneumonia.

Keywords: Case ascertainment; data analysis; empirical research; hospital admissions; influenza; seasonal and pandemic; statistical models.

Figure 1

Respiratory admissions to hospital for urgent care, Discharge Abstract Database participating hospitals (Canada, excl Quebec) showing model fit and estimated baseline. (A) Weekly respiratory admissions to hospital for persons aged 20–49 years (). The impact of the pandemic strain on younger adults is obvious; the shaded area represents excess admissions attributed seasonal and pandemic influenza, corresponding to annual rates of 11/100 000 and 39/100 000, respectively. The estimated baseline curve () accounts for seasonality and secular trends inherent in all respiratory admissions for this age group. Seasonality in this population was not well characterized by the sinusoidal function, and a noticeable spike over the extended Christmas holiday period into the 1st week of January can be observed. There was no increase in baseline respiratory admissions in this age group over the pandemic period. Model predicted values () correspond closely to the actual number of admissions (). (B) Weekly admissions to hospital with any mention of J11 (influenza, virus not identified). The weekly number of admissions is shown on a log scale to highlight the characteristics of the model estimated baseline. The baseline () corresponds to the expected number of background admissions coded to J11 [diagnosed as influenza‐like illness (ILI)] – that is ILI admissions due to other viruses. Once the pandemic was announced, J11 admissions increased despite efforts to test all suspected H1N1/2009 admissions. In addition to significant increases in admissions likely due to H1N1/2009, the fitted model suggests that there was also a significant increase in the diagnosis of ILI among persons admitted with other acute respiratory infections, as a large part of the increase was not associated with the level of H1N1/2009 activity (jump in baseline starting in May 2009). The model predicted number of J11 admissions closely follows the actual number of J11 admissions. Note that while the use of a log scale was helpful to illustrate variation in the estimated baseline, it also distorted the visual perception of the disease burden.

Figure 2

Respiratory admissions to hospital for urgent care, Discharge Abstract Database participating hospitals (Canada, excl Quebec) by presence of pneumonia. The weekly number of admissions (), model estimated baseline (), and the weekly number of admissions predicted by the model () are shown for respiratory admissions without any mention of influenza or influenza‐like illness (ILI) and with (A) or without pneumonia (B). The area between the predicted and baseline curves is shown below () and corresponds to the number of admissions attributed to influenza, but for which a diagnosis of influenza or ILI was not recorded in the patient’s chart. During the fall pandemic wave, case ascertainment appears to have been nearly complete for patients with respiratory conditions other than pneumonia.

Figure 3

Hospitalization rates and multipliers for urgent care 2003/04–2009/10, Discharge Abstract Database participating hospitals (Canada, excludes Quebec), by age group. (A) Age‐specific hospitalization rates for admissions with any mention of an identified influenza virus (J09, J10) are compared on a log scale with rates for admissions attributed to influenza for the seasonal and pandemic periods. In younger age groups, the influenza‐attributed rates were based on excess respiratory admissions excluding asthma and bronchiolitis. (B) Multipliers by 5‐year age group: admissions to hospital attributed to H1N1/2009 divided by J09 or J10 coded admissions, Canada. The multipliers increased with increasing age. (C) Ratio of the influenza‐attributed rates for the pandemic period to the average for the seasonal period (2003/04–2008/09) is shown for 5‐year age groups (–▲–). The corresponding rate ratio for the H1N1/2009 pandemic to the average annual rate for seasonal H1N1 was calculated for the following age groups: <50, 50–64, 65–74, and 75 years or older (–– for each age group).