Monthly respiratory deaths per 10,000 people in three representative provinces of Spain, Jan-1915 to June-1919. The black curve is the monthly number of respiratory deaths. Shaded areas highlight time periods of high mortality associated with the 1918–1919 pandemic in Spain. The Serfling seasonal regression model baseline (blue curve) and corresponding upper limit of the 95% confidence interval of the baseline (red curve) are also shown. Excess deaths are above the upper limit of the baseline mortality curve calibrated using mortality levels prior to the 1918 influenza pandemic.