circulatory mortality rates during the influenza season, with 95% confidence intervals (CIs). The seasonal baseline represents the expected number of deaths in the absence of influenza virus circulation. For Uruguay, data from the 2004 time series were added directly to the 2007--2009 series to provide a 4-year time series for analysis.

For the United States, influenza-associated deaths were estimated using negative binomial regression models as previously described.[@b5],[@b19]

Defining the influenza epidemic period
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To arrive at influenza-associated deaths, we defined annual periods of influenza circulation. Excess mortality in any one season was defined as the number of deaths occurring above the baseline during the influenza epidemic periods. The influenza period was defined in two different ways depending on the country.

### Pneumonia and influenza deaths

For Argentina, Chile, Mexico, southern Brazil, and Uruguay, viralregion during 2002--2008 and that 81% of all deaths occurred among persons aged ≥65 years, of which the majority occurred among the ≥75-year age group, who experience the highest rate of influenza mortality, almost five times that of the 65--74 age group. Forty thousand influenza deaths were respiratory-related and 45 000 deaths were circulatory-related.

We also found that year-to-year variability in excess influenza mortality was larger than differences between countries. In any one calendar year, the influenza-specific mortality estimates generated using traditional techniques correlated very poorly between the seven countries with available data. Other than Paraguay and Mexico, which implemented influenza vaccination in 2004 and 2005, respectively, the remaining five countries used vaccine throughout the study period.[@b25] Moreover, vaccine coverage cannot explain the differences between countries. For instance, both Argentina and