During the 2009 influenza pandemic, weekly mortality data were analyzed to estimate excess mortality above a seasonally adjusted baseline modeled from prior years' data.

Between the 1962-1963 and 2008-2009 seasons, among persons ≥ 25 years old, excess mortality had been substantially higher during influenza A(H3N2)-dominant years than during A(H1N1)-dominant years. Among persons ≥ 15 years of age, excess mortality was higher in the 1968-1969 influenza pandemic season than during any other season. During the 2009-2010 pandemic, among all age groups <65 years old, excess mortality increased earlier than during any of the previous 47 seasons, eventually exceeding mortality in any prior non-pandemic season. In the ≥ 65-year-old age group, excess mortality remained relatively low, at rates typical of seasonal influenza A(H1N1) seasons.

The model provided a timely assessment of severity during the 2009-2010 influenza pandemic, showing that, compared with prior seasons, mortality was relatively high among persons <65 years old and relatively low among those ≥ 65 years old.