Mini Oral Australian Epidemiology Association ASM 2018

Improving understandings of Victorian influenza epidemiology: Inter-seasonal standardisation of laboratory-confirmed notifications (#24)

Elenor Kerr 1 , James E Fielding 2 , Robert Moss 1
  1. Melbourne School of Population and Global Health, The University of Melbourne, Melbourne, Vic, Australia
  2. Victorian Infectious Disease Reference Laboratory , The Doherty Institute for Infection and Immunity, Melbourne, Vic, Australia

Background

The value of notifiable laboratory-confirmed influenza surveillance is currently limited in Victoria because it does not account for increases in testing. Disproportionate increases in laboratory-confirmed influenza notifications since 2009 are likely driven by expanded PCR testing1,2. However, with insufficient access to negative or total testing data, there is currently no statistical method to account for testing increases. 

Aim

The study aimed to determine whether utilising an inter-seasonal standardisation model could account for increases in testing and therefore provide greater understanding of Victoria’s influenza epidemiology.

Methods

Standardisation of Victorian Notifiable Disease Surveillance System notification data from 2010 to 2017 was explored with two models of inter-seasonal testing multipliers. Models were established by identifying inter-seasonal periods and deriving variations in average notifications from reference indicator periods (2010-2011, 2011-2012). Models were applied to seasonal notifications with fit determined against observed annual influenza-like-illness trends as per Victorian Sentinel Practice Influenza Network and National Home Doctor Service data.

Results

Standardised notifications using the 2010-2011 baseline were discordant with trends for all years except 2013. Standardisations utilising the 2011-2012 baseline approximately reflected trends for 2013, 2014 and 2016. Neither standardisation produced comparable results for 2015 and 2017.

Conclusion

As a method inter-seasonal standardisation has limited ability to account for substantial between and within season changes. Total testing or negative test result data remain crucial to restoring the value of Victorian notifications data. However, inter-seasonal standardisation can somewhat account for increases in testing and provided useful insights into increases in testing and inter-seasonal periods. 

  1. Lambert SB, Faux CE, Grant KA, Williams SH, Bletchly C, Catton MG, Smith DW & Kelly HA. Influenza Surveillance in Australia: we need to do more than count. Medical Journal of Australia. 2010; 193(1): 43-45.
  2. Fielding JE, Regan AK, Dalton CB, Chilver MBN & Sullivan SG. How severe was the 2015 influenza season in Australia? Medical Journal of Australia. 2016; 204(2): 60-61.