May Chiew1, Heather F Gidding2, Aditi Dey1, James Wood2, Nicolee Martin3, Stephanie Davis4, Peter McIntyre1. 1. National Centre for Immunisation Research and Surveillance of Vaccine Preventable Diseases, The Children's Hospital Westmead, Sydney, Australia . 2. School of Public Health and Community Medicine, University of New South Wales, Samuels Bldg, Botany St, Randwick, Sydney 2052, Australia . 3. Vaccine Preventable Disease Surveillance, Department of Health, Canberra, Australia . 4. National Centre for Epidemiology and Population Health, Australian National University, Canberra, Australia .
Abstract
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. FINDINGS: 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. CONCLUSION: The fact that consistent estimates of R were obtained from all three methods enhances confidence in the validity of these methods for determining R.
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. FINDINGS: 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. CONCLUSION: The fact that consistent estimates of R were obtained from all three methods enhances confidence in the validity of these methods for determining R.
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