Association of influenza epidemics in France and the USA with global climate variability

Background: The reasons for the seasonality and annual changes in the impact of influenza epidemics remain poorly understood. A strong coherence of influenza epidemics at a hemispheric level may suggest the role of global factors, such as climate, as a driving force of seasonality. The El Nino Southern Oscillation (ENSO) represents the largest signal in inter-annual climate variation, affecting global atmospheric and oceanic circulation patterns. The phenomenon undergoes cycles between warm ENSO conditions (which are extreme during El Nino episodes) and cold ENSO conditions (extreme during La Nina episodes). Materials and methods: We investigated the covariations between ENSO and the impact of influenza as measured by the number of pneumonia and influenza (P&I) excess deaths in France and the USA during the winter epidemics of 1971-1997. Results: On average in France (60.2 (M) million inhabitants in 2003), 2500 P&I excess deaths occurred per season (range 0-9500) during the 1971-1997 period. An average of 6215 excess deaths (range 0-13,600) occurred in the USA (290.3 M inhabitants in 2003). In both countries, the number of P&I excess deaths was significantly higher during the 10 seasons with cold ENSO conditions (mean+/-S.E.; 3530+/-654 excess deaths in France; 8290+/-900 in the USA) than during the 16 seasons with warm ENSO conditions (1856+/-574; 4919+/-977) (Wilcoxon rank test for France P=0.05 and for the USA P=0.03). Conclusion: These findings suggest that the impact of influenza epidemics, in terms of excess mortality, is associated with the ENSO conditions. An understanding of the mechanisms responsible for this association could lead to improved early warning and better control of influenza. (C) 2004 Published by Elsevier B.V.