Mortality burden of the 2009-10 influenza pandemic in the United States: improving the timeliness of influenza severity estimates using inpatient mortality records

Abstract

Background: Delays in the release of national vital statistics hinder timely assessment of influenza severity, especially during pandemics. Inpatient mortality records could provide timelier estimates of influenza-associated mortality.

Methods: We compiled weekly age-specific deaths for various causes from US State Inpatient Databases (1990-2010) and national vital statistics (1990-2009). We calculated influenza-attributable excess deaths by season based on Poisson regression models driven by indicators of respiratory virus activity, seasonality, and temporal trends.

Results: Extrapolations of excess mortality from inpatient data fell within 11% and 17% of vital statistics estimates for pandemic and seasonal influenza, respectively, with high year-to-year correlation (Spearman's rho = 0.87-0.90, P < 0.001, n = 19). We attribute 14,800 excess respiratory and cardiac deaths (95% CI: 10,000-19,650) to pandemic influenza activity during April 2009-April 2010, 79% of which occurred in people under 65 years.

Conclusions: Modeling inpatient mortality records provides useful estimates of influenza severity in advance of national vital statistics release, capturing both the magnitude and the age distribution of pandemic and epidemic deaths. We provide the first age- and cause-specific estimates of the 2009 pandemic mortality burden using traditional 'excess mortality' methods, confirming the unusual burden of this virus in young populations. Our inpatient-based approach could help monitor mortality trends in other infectious diseases.

Keywords: Age patterns; influenza; influenza-related mortality; pandemic influenza; years of life lost.

Figure 1

Weekly deaths per 100 000 due to (A) pneumonia and influenza, (B) respiratory causes and (C) respiratory and cardiac causes in vital statistics (NCHS, black) and inpatient mortality (SID, red) data. Legends display week‐to‐week correlations between the two datasets based on Spearman's rank correlation coefficient, rho. NCHS: National Center for Health Statistics. SID: State Inpatient Database.

Figure 2

Comparison of seasonal excess mortality rates derived from vital statistics and SID inpatient mortality datasets for 19 pre‐pandemic seasons 1990/91–2008/09, by death outcome (black dots: estimated excess deaths; blue line: linear model; red dashed line: 95% confidence intervals. left panel: P&I: Pneumonia and Influenza; middle panel: Resp: Respiratory; right panel: R&C: Respiratory and Cardiac). Model R ² values are displayed in the legends. Model predictions for the pandemic based on inpatient‐derived estimates for the April–December 2009 period are depicted as blue dots. Estimates for the April 2009–April 2010 period are depicted as red dots. For comparison, vital statistics‐derived excess mortality estimates for the April‐December 2009 period are depicted as green squares. Estimated slopes (units: NCHS excess death rate/SID excess death rate) for the simple linear models are 0·71 (P&I), 0·74 (Resp), and 1·4 (R&C). NCHS: National Center for Health Statistics. SID: State Inpatient Database.

Figure 3

Weekly inpatient mortality rates due to respiratory causes in (A) under 5‐year‐olds, (B) 5‐ to 24‐year‐olds, (C) 25‐ to 44‐year‐olds, (D) 45‐ to 64‐year‐olds, and (E) over 65 years, 2003/04–2009/10 seasons. 3‐week moving averages of observed mortality rates are depicted in black. Model‐fitted values are depicted in blue, and seasonal baselines (adjusted for circulation of respiratory syncytial virus) are depicted in red. The vertical dashed line demarcates the onset of A/H1N1pdm circulation.

Figure 4

Percentage of excess respiratory deaths occurring in each, by season and database. NCHS: National Center for Health Statistics. SID: State Inpatient Database. Data labeled ‘past seasons’ represents the median, 1st and 3rd quartiles of percentages of influenza‐related deaths occurring in each age group in the pre‐pandemic period, 2003/2004 to 2008/2009. For the pandemic season, we considered the full pandemic period April 2009–April 2010 for the inpatient mortality estimates, and the truncated pandemic season April–December 2009 for vital statistics estimates. Error bars during the pandemic period depict 95% confidence intervals derived from age‐specific excess mortality models.