Estimating the measles effective reproduction number in Australia from routine notification data.
OBJECTIVE
To estimate the measles effective reproduction number (R) in Australia by modelling routinely collected notification data.


METHODS
R was estimated for 2009-2011 by means of three methods, using data from Australia's National Notifiable Disease Surveillance System. Method 1 estimated R as 1 - P, where P equals the proportion of cases that were imported, as determined from data on place of acquisition. The other methods estimated R by fitting a subcritical branching process that modelled the spread of an infection with a given R to the observed distributions of outbreak sizes (method 2) and generations of spread (method 3). Stata version 12 was used for method 2 and Matlab version R2012 was used for method 3. For all methods, calculation of 95% confidence intervals (CIs) was performed using a normal approximation based on estimated standard errors.


RESULTS
During 2009-2011, 367 notifiable measles cases occurred in Australia (mean annual rate: 5.5 cases per million population). Data were 100% complete for importation status but 77% complete for outbreak reference number. R was estimated as < 1 for all years and data types, with values of 0.65 (95% CI: 0.60-0.70) obtained by method 1, 0.64 (95% CI: 0.56-0.72) by method 2 and 0.47 (95% CI: 0.38-0.57) by method 3.


CONCLUSIONS
The fact that consistent estimates of R were obtained from all three methods enhances confidence in the validity of these methods for determining R.
National Centre for Immunisation Research and Surveillance of Vaccine Preventable Diseases, The Children's Hospital Westmead, Sydney, Australia .
May
M
Chiew
School of Public Health and Community Medicine, University of New South Wales, Samuels Bldg, Botany St, Randwick, Sydney 2052, Australia .
Heather F
HF
Gidding
National Centre for Immunisation Research and Surveillance of Vaccine Preventable Diseases, The Children's Hospital Westmead, Sydney, Australia .
Aditi
A
Dey
School of Public Health and Community Medicine, University of New South Wales, Samuels Bldg, Botany St, Randwick, Sydney 2052, Australia .
James
J
Wood
Vaccine Preventable Disease Surveillance, Department of Health, Canberra, Australia .
Nicolee
N
Martin
National Centre for Epidemiology and Population Health, Australian National University, Canberra, Australia .
Stephanie
S
Davis
National Centre for Immunisation Research and Surveillance of Vaccine Preventable Diseases, The Children's Hospital Westmead, Sydney, Australia .
Peter
P
McIntyre
D001315:Australia / Q000453:epidemiology; D018563:Disease Notification / Q000379:methods / Q000592:standards*; D004196:Disease Outbreaks / Q000706:statistics & numerical data; D006801:Humans; D016013:Likelihood Functions; D008457:Measles / Q000453:epidemiology*; D062486:Public Health Surveillance / Q000379:methods; D018571:Sentinel Surveillance; D014195:Travel