Abstract
BackgroundInfluenza epidemics have a substantial impact on human health, by increasing the mortality from pneumonia and influenza, respiratory and circulatory diseases, and all causes. This paper provides estimates of excess mortality rates associated with influenza virus circulation for 7 causes of death and 8 age groups in Portugal during the period of 1980–2004.Methodology/Principal FindingsWe compiled monthly mortality time series data by age for all-cause mortality, cerebrovascular diseases, ischemic heart diseases, diseases of the respiratory system, chronic respiratory diseases, pneumonia and influenza. We also used a control outcome, deaths from injuries. Age- and cause-specific baseline mortality was modelled by the ARIMA approach; excess deaths attributable to influenza were calculated by subtracting expected deaths from observed deaths during influenza epidemic periods. Influenza was associated with a seasonal average of 24.7 all-cause excess deaths per 100,000 inhabitants, approximately 90% of which were among seniors over 65 yrs. Excess mortality was 3–6 fold higher during seasons dominated by the A(H3N2) subtype than seasons dominated by A(H1N1)/B. High excess mortality impact was also seen in children under the age of four years. Seasonal excess mortality rates from all the studied causes of death were highly correlated with each other (Pearson correlation range, 0.65 to 0.95, P<0.001) and with seasonal rates of influenza-like-illness (ILI) among seniors over 65 years (Pearson correlation rho>0.64, P<0.05). By contrast, there was no correlation with excess mortality from injuries.Conclusions/SignificanceOur excess mortality approach is specific to influenza virus activity and produces influenza-related mortality rates for Portugal that are similar to those published for other countries. Our results indicate that all-cause excess mortality is a robust indicator of influenza burden in Portugal, and could be used to monitor the impact of influenza epidemics in this country. Additional studies are warranted to confirm these findings in other settings.
Highlights
Influenza is an acute respiratory viral infection which typically occurs in the winter months in temperate areas
1980–2004, the seasonal average number of all cause excess deaths associated with influenza epidemics was 2,475 in Portugal, 90% of which occurred in people aged $65 years, representing a crude excess all-cause death rate of 26 per 100,000
The age-standardized mortality rates was 3.1/ 100,000 for respiratory diseases, 1.5/100,000 for pneumonia and Epidemic periods based on the Portuguese Influenza Surveillance System*
Summary
Influenza is an acute respiratory viral infection which typically occurs in the winter months in temperate areas. Seasonal influenza epidemics have a substantial mortality and morbidity impact on human health globally [1,2,3]. Excess morbidity is calculated in much the same way Statistical models, such as Poisson regression [11], multiple linear regression [12], cyclical regression [13], and autoregressive integrated moving average (ARIMA) [1], have been used to estimate excess mortality and take into account factors that are independent of influenza activity, such as seasonal variations and temporal trends in baseline mortality. Influenza epidemics have a substantial impact on human health, by increasing the mortality from pneumonia and influenza, respiratory and circulatory diseases, and all causes. This paper provides estimates of excess mortality rates associated with influenza virus circulation for 7 causes of death and 8 age groups in Portugal during the period of 1980– 2004
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