Mortality Attributable to Seasonal and Pandemic Influenza, Australia, 2003 to 2009, Using a Novel Time Series Smoothing Approach

Abstract

BackgroundOfficial statistics under-estimate influenza deaths. Time series methods allow the estimation of influenza-attributable mortality. The methods often model background, non-influenza mortality using a cyclic, harmonic regression model based on the Serfling approach. This approach assumes that the seasonal pattern of non-influenza mortality is the same each year, which may not always be accurate.AimTo estimate Australian seasonal and pandemic influenza-attributable mortality from 2003 to 2009, and to assess a more flexible influenza mortality estimation approach.MethodsWe used a semi-parametric generalized additive model (GAM) to replace the conventional seasonal harmonic terms with a smoothing spline of time (‘spline model’) to estimate influenza-attributable respiratory, respiratory and circulatory, and all-cause mortality in persons aged <65 and ≥65 years. Influenza A(H1N1)pdm09, seasonal influenza A and B virus laboratory detection time series were used as independent variables. Model fit and estimates were compared with those of a harmonic model.ResultsCompared with the harmonic model, the spline model improved model fit by up to 20%. In <65 year-olds, the estimated respiratory mortality attributable to pandemic influenza A(H1N1)pdm09 was 0.5 (95% confidence interval (CI), 0.3, 0.7) per 100,000; similar to that of the years with the highest seasonal influenza A mortality, 2003 and 2007 (A/H3N2 years). In ≥65 year-olds, the highest annual seasonal influenza A mortality estimate was 25.8 (95% CI 22.2, 29.5) per 100,000 in 2003, five-fold higher than the non-statistically significant 2009 pandemic influenza estimate in that age group. Seasonal influenza B mortality estimates were negligible.ConclusionsThe spline model achieved a better model fit. The study provides additional evidence that seasonal influenza, particularly A/H3N2, remains an important cause of mortality in Australia and that the epidemic of pandemic influenza A (H1N1)pdm09 virus in 2009 did not result in mortality greater than seasonal A/H3N2 influenza mortality, even in younger age groups.

Mortality burden from seasonal influenza and 2009 H1N1 pandemic influenza in Beijing, China, 2007‐2013

“…Consistent with studies conducted in other countries, our study found that, compared with seasonal epidemics, there was an “age shift” in age‐specific mortality burden toward persons <65 years of age during the influenza A(H1N1)pdm09 pandemic period 4, 20, 29, 33, 34, 35, 36. A previous meta‐analysis from 27 studies showed that the influenza A(H1N1)pdm09 cumulative incidence varied significantly by age with much higher incidence in younger people 37.…”

Influenza‐associated mortality in Yancheng, China, 2011‐15

“…This corresponded to 4.6% of all respiratory deaths during the study period, again consistent with what has been reported elsewhere, for example, 6.0% in Hong Kong,4 2% in the United States,17 3.2% in Australia,18 and 4.1% in Bangkok 10. Variation in this fraction would be affected to some degree by differences in coding practices between locations 6.…”

Burden of influenza, respiratory syncytial virus, and other respiratory viruses and the completeness of respiratory viral identification among respiratory inpatients, Canada, 2003‐2014

“…Because regression estimates are not available in real time, researchers have proposed other methods10, 11 to use surveillance data to predict the full burden in near real time, only some of which have been evaluated at year‐end against a regression approach 6, 12, 13, 14…”