What was done
The authors used a Poisson regression model with regression splines to control for nonlinear confounders to analyze a time series of all-cause mortality in nine French cities before, during and after the August 2003 European heatwave.

What was learned
Le Tertre et al. report that "after controlling for long-term and seasonal time trends and the usual effects of temperature and air pollution, we estimated that 3,096 extra deaths resulted from the heatwave," with the daily risk of dying during the heatwave rising by 16% in Le Havre and by as much as 400% in Paris. In addition, they say "there was little evidence of mortality displacement in the few weeks after the heatwave, with an estimated deficit of 253 deaths at the end of the period."

What it means
With respect to "the lack of evidence of substantial short-term mortality displacement," the fourteen French researchers say "this finding appears to be a unique characteristic of the 2003 heatwave in France," because "a harvesting effect is often observed with high temperatures" and "mortality during the last months of 2003 was not lower than during a control period (Hemon and Jougla, 2004)." However, they also note that "mortality during the first months of year 2004 was much lower than during a control period (Valleron and Bourmendil, 2004), and that "an analysis of 2004 mortality statistics in France (Pison, 2005) shows that the decrease in mortality during 2004 was larger than the increase due to the heatwave in 2003 [our italics]."

These observations suggest to us that the intensity of the 2003 heatwave may have been great enough, and its duration long enough, to cause the immediate deaths of people who would not have died for perhaps as much as a year or more after its occurrence. Hence, the mortality harvesting effect characteristic of high temperatures may still have been operative in the case of the 2003 heatwave and even more powerfully than is typically observed, bringing about the deaths of people who might well have lived a year or more longer in its absence ... maybe.

References
Hemon, D. and Jougla, E. 2004. Surmortalite liee a la canicule d'aout 2003. Suivi de la mortalite (21 aout-31 decembre) et causes medicales des deces (1-20 aout 2003). INSERM: 1-76.

Pison, G. 2004. France 2004: l'experance de vie franchit le seuil de 80 ans. Population et societes. Ined (410).

Valleron, A.J. and Boumendil, A. 2004. Epidemiologies et canicules: analyses de la vague de chaleur 2003 en France. C R Biol. 327:1125-1141.