Epidemiology and Infection

Original Papers

Epidemic studies

Quantifying differences in the epidemic curves from three influenza surveillance systems: a nonlinear regression analysis

E. G. THOMASa1 c1, J. M. McCAWa1a2, H. A. KELLYa3a4, K. A. GRANTa3 and J. McVERNONa1a2

a1 Centre for Epidemiology and Biostatistics, Melbourne School of Population and Global Health, University of Melbourne, Victoria, Australia

a2 Vaccine and Immunisation Research Group, Murdoch Childrens’ Research Institute, Victoria, Australia

a3 Epidemiology Unit, Victorian Infectious Diseases Reference Laboratory, Victoria, Australia

a4 National Centre for Epidemiology and Population Health, Australian National University, ACT, Australia


Influenza surveillance enables systematic collection of data on spatially and demographically heterogeneous epidemics. Different data collection mechanisms record different aspects of the underlying epidemic with varying bias and noise. We aimed to characterize key differences in weekly incidence data from three influenza surveillance systems in Melbourne, Australia, from 2009 to 2012: laboratory-confirmed influenza notified to the Victorian Department of Health, influenza-like illness (ILI) reported through the Victorian General Practice Sentinel Surveillance scheme, and ILI cases presenting to the Melbourne Medical Deputising Service. Using nonlinear regression, we found that after adjusting for the effects of geographical region and age group, characteristics of the epidemic curve (including season length, timing of peak incidence and constant baseline activity) varied across the systems. We conclude that unmeasured factors endogenous to each surveillance system cause differences in the disease patterns recorded. Future research, particularly data synthesis studies, could benefit from accounting for these differences.

(Received November 27 2013)

(Revised March 07 2014)

(Accepted March 09 2014)

(Online publication April 23 2014)

Key words

  • Influenza;
  • spatial;
  • surveillance;
  • surveillance system