Decline of Influenza Mortality in Canada since the Spanish Flu

Influenza has long been an important source of mortality worldwide. However, the historical patterns of death due to this disease are still poorly understood. We present the first analysis of the long-term patterns of annual influenza mortality in Canada, covering nearly the entire period since the Spanish flu pandemic of 1918. The death rate due to influenza showed a clear exponential decay from 1922 (the earliest year accurate statistics are available) until 2003. In addition, a log transformation of the influenza specific death rate data revealed a rise in influenza mortality above the long-term trend during 1998-2000. There was no strong evidence of periodicity in influenza mortality, with the time series showing only weak positive autocorrelation. We compare the decline in influenza mortality to historical patterns in death due to pneumonia and other infectious respiratory diseases. The observed decay in influenza deaths could have been due, in part, to improvements in population health, public health practice, and the treatment of infectious diseases. However, we also argue that such a decline may have been due to improved, long-lasting, individual and population-level immunity to influenza.