Measuring Australia's excess mortality by remoteness areas during the COVID-19 pandemic until December 2023

Measuring Australia's excess deaths by remoteness during the COVID-19 pandemic until December 2023

Released
28/02/2025

Key statistics

Overview

This article provides weekly and annual excess mortality estimates for deaths occurring in Australia by remoteness areas until the end of December 2023.

The ABS has produced several reports on excess mortality during the COVID-19 pandemic. These provide statistics for Australia and states and territories. The most recent report provided estimates until the end of 2023. This article provides extra geographic detail for the first time, presenting excess mortality by ‘remoteness areas’ (i.e. for major cities, inner regional, outer regional, remote, and very remote parts of Australia). 

Excess mortality is typically defined as the difference between the observed number of deaths in a specified time period and the expected numbers of deaths in that same time period. Excess mortality measures can account for deaths due to COVID-19, potentially misclassified or undiagnosed COVID-19 deaths, and other mortality that may be indirectly related to the pandemic (e.g. relating to social isolation or changed access to health care). 

This data is designed to provide further insights to excess mortality estimates previously produced by the ABS so it uses the same methods as those earlier reports. It’s designed to answer the research question: 'How does the number of deaths which has occurred since the beginning of the COVID-19 pandemic (2020-2023) compare to the number of deaths expected had the pandemic not occurred?'. Please see the Interpreting Results section in the previous article for more information.

Key points:

  • For all remoteness areas, the highest excess mortality was in 2022. This ranged from 3.4% in outer regional Australia to 13.4% in very remote Australia.
  • Excess mortality in all remoteness areas more than halved in 2023 compared to 2022. In outer regional Australia there was negative excess mortality of 1.1% (meaning there were fewer deaths than expected). The highest excess mortality in 2023 occurred in major cities (5.2%).
  • All remoteness areas except remote Australia experienced negative excess mortality in 2020. Inner regional and outer regional Australia also experienced negative excess mortality in 2021. The highest excess mortality in 2021 occurred in remote Australia (4.6%).

The following table provides a summary of excess mortality estimates by remoteness area expressed as the percentage above expected mortality for the years 2020-2023. Excess mortality estimates for Australia are included as a reference point.

Excess mortality as a percentage above expected by remoteness area, 2020-23
2020202120222023
Australia-3.11.611.75.1
Major Cities-2.52.412.95.2
Inner Regional-4.4-0.38.63.6
Outer Regional-5.3-1.73.4-1.1
Remote0.24.611.54.2
Very Remote-4.32.913.42.5

Remoteness areas in Australia

The Australian Statistical Geography Standard (ASGS) Remoteness Structure divides Australia into five classes of remoteness which are characterised by a measure of relative geographic access to services. These five classes are Major cities of Australia, Inner regional Australia, Outer regional Australia, Remote Australia and Very remote Australia. The map below illustrates the relative size and location of the different remoteness areas. 

Over 70% of Australia's population live in major cities (see Regional population). Nearly 20% live in inner regional areas, and under 10% live in outer regional areas. Just over 1% of Australia's population are in remote Australia, and under 1% live in very remote Australia. As the remoteness area structure relates to access to services not every state and territory has all five classes of remoteness included. For example, the Northern Territory and Tasmania have no areas considered as a major city (a capital city differs from a major city) and the Australian Capital Territory has no areas considered remote and very remote. 

Map of ASGS Edition 3 Remoteness Areas for Australia

Map detailing the five different Remoteness Area classes which make up the Remoteness Structure. The map shows areas shaded in light to dark green colours, to demonstrate Very Remote Australia, Remote Australia, Outer Regional Australia, Inner Regional Australia and Major Cities of Australia respectively.

Map detailing the five different Remoteness Area classes which make up the Remoteness Structure. The map shows areas shaded in light to dark green colours, to demonstrate Very Remote Australia, Remote Australia, Outer Regional Australia, Inner Regional Australia and Major Cities of Australia respectively.

All-cause mortality rates by remoteness areas

Health status can differ by remoteness areas. Geographic location can affect access to health services (see Rural and remote health, AIHW, 2024). The graph below shows that age-standardised mortality rates are higher in rural and remote areas. Mortality rates increase with increasing remoteness. This trend has been consistent over time. The mortality rate of those living in very remote areas has consistently been over 1.4 times higher than those living in major cities over the last decade. 

a. Age-standardised death rate (SDR). Death rate per 100,000 standard estimated resident population as at 30 June.
b. Remoteness classification is based on area of usual residence. 

COVID-19 associated mortality rates by remoteness areas

COVID-19 associated mortality rates have varied across remoteness areas across the pandemic. The table below shows that for 2020 and 2021 age-standardised COVID-19 mortality rates were highest in major cities. During these years there were five deaths associated with the virus in remote and very remote areas (a mortality rate was not produced for remote areas in these years due to the very small number of deaths). In 2020, there were zero COVID-19 associated deaths in very remote Australia. This changed in 2022 when those in very remote areas recorded the highest COVID-19 mortality rates. In 2022 and 2023, the rates were highest in very remote Australia. In 2023 the trend of COVID-19 associated mortality is the same as the trend for all-cause mortality. The rates increase with increasing remoteness (so rates are highest in very remote areas and lowest in major cities). 

Age standardised death rates from or with COVID-19 by remoteness areas, Australia, 2020-2023
2020202120222023
Major cities3.65.839.016.2
Inner regional0.31.132.316.6
Outer regionalnpnp31.116.9
Remotenpnp34.519.9
Very remote0np41.920.3

a. Age-standardised death rate (SDR). Death rate per 100,000 standard estimated resident population as at 30 June.
b. SDRs based on small numbers are volatile and unreliable. SDRs based on less than 20 deaths have not been published. 
c. Remoteness classification is based on area of usual residence. 
d. COVID-19 associated deaths (also called ‘deaths from or with COVID-19’) are any deaths where COVID-19 has directly caused or significantly contributed to death.

Measuring excess mortality

Across the world, health and statistical authorities have sought to measure excess mortality during the COVID-19 pandemic. Different methodologies have been applied, with the goal to predict an expected number of deaths for a given year. Choice of model and baseline can markedly affect estimates of expected (and therefore excess) mortality. Estimating the expected future seasonality for deaths can be a challenge in many models. The suitability of a model can depend on factors such as country context, data quality and collection methods.

The ABS has adopted aspects of a methodology used by New South Wales (NSW) Health, applying a cyclical linear regression with a robust estimation procedure to produce both an expected number of deaths and a range of expected deaths. The ABS has applied this model to estimate age specific death rates (ASDRs) for certain age groups, and converted the expected death rates into an expected number of deaths for each age group. The ABS then added these across age groups to obtain an expected count for the total population. Using ASDRs accounts for changes in population size and age composition. For more information, please see the Methodology section. 

Interpreting results

Considerations for interpreting information in this report

Outputs from excess mortality estimates will differ depending on the calculation applied and the scope of the input data. When interpreting the results in this report the following factors must be taken into consideration. 

  • This report highlights weeks where excess deaths are statistically significant. In any given period, even if no temporary health hazards influence the number of deaths (such as community transmission of influenza or COVID-19), there is some natural variation in patterns of mortality. While the actual number of deaths may be different from the expected number of deaths, it should fall within an expected range (for the statistics in this article, there is a 95% chance that the expected number of deaths would lie between the upper and lower bounds of the confidence intervals). When actual observations (counts of death) exceed the upper threshold or drop below the lower threshold this indicates a statistically significant change in the pattern of mortality. The expected range should be used in conjunction with the percentage of excess mortality.
  • A single week above threshold does not necessarily suggest statistically significant excess mortality. Prolonged periods (2 or more weeks) where counts are outside thresholds suggest more strongly that the numbers of deaths are above or below normal. 
  • Data is reported by date of death occurrence. Remoteness area analysis includes all deaths that occurred by 31 December 2023 and registered by 31 October 2024. Data may change as additional registrations are received by the ABS. For the Australia level data, the scope is deaths that occurred by 31 December 2023 and were registered and received by the ABS by 31 March 2024. This is because Australia level data is used as a reference point only and matches previously published counts. It is expected that Australia level data would be close to complete and any additional deaths received would make a limited difference to high level counts and estimates. Please refer to the ‘Timeliness and completeness of the data’ section of Provisional Mortality Statistics for more information. 
  • The number of deaths by remoteness areas will not add to the Australian total. This is because some deaths can’t be assigned a remoteness area, for example because not enough address information is supplied to the ABS to identify the remoteness area. 
  • Reported deaths from or with COVID-19 are identified from death certificates or coroner reports as part of the death registration process. There may be some deaths where COVID-19 was a contributing factor but it was not recorded on the death certificate (for example, the medical practitioner may be unaware of a present or past infection). If COVID-19 is not recorded on the death certificate it is not included in COVID-19 death tabulations presented. 
  • The number of deaths associated with COVID-19 is the combination of deaths attributed as being due to COVID-19 (the virus caused complications leading directly to death) and deaths where COVID-19 contributed to death (these are deaths where there was another pathway to death, but the virus severely impacted the health of the deceased). 

Weekly all-cause mortality: Australia

The ABS has previously published excess mortality at the national level in Measuring Australia's excess mortality during the COVID-19 pandemic until December 2023). It is included again in this article as a reference point for excess mortality estimates for the remoteness areas. 

  • Excess mortality for Australia in 2023 was estimated at 5.1% above expected. This is a decrease compared to 2022, when excess mortality was 11.7%.  
  • COVID-19 associated deaths (as identified on death certificates) were a key contributor to excess mortality in Australia from late 2021 through to 2023. Excess mortality during this period corresponded with peaks in COVID-19 waves. 
  • Deaths were significantly lower than expected from the week beginning 1 June to mid-July 2020, dropping below lower thresholds. Winter months are typically associated with higher mortality. These decreases provide insights into how public health measures put in place to manage the COVID-19 pandemic impacted mortality. 

     

Excess mortality by year, Australia, 2020-23
2020202120222023
Expected170,046169,048170,933173,121
Observed164,805171,823190,955181,865
Excess-5,2412,77520,0228,744
% Excess-3.11.611.75.1
Reported deaths from or with COVID-199161,44813,3175,952

a. Data is provisional and subject to change.
b. Years are based on a sum of International Organization for Standardisation (ISO) weeks. Weeks start on Monday, and week 1 always contains the 4th of January. There are 53 weeks in 2020. There are 52 weeks in 2021, 2022 and 2023. 
c. Reported deaths 'from' or 'with' COVID-19 are as recorded on the death certificate. 
d. Includes all deaths (both doctor and coroner certified) that occurred by 31 December 2023 and were registered and received by the ABS by 31 March 2024.

a. Data is provisional and will change as additional death registrations are received.
b. The baseline period generating the regression model is from 2013 to 2019. The prediction window begins in 2020. 
c. Includes all deaths (both doctor and coroner certified) that occurred by 31 December 2023 and were registered and received by the ABS by 31 March 2024. 

Weekly all-cause mortality: Major cities

  • Excess mortality for major cities was highest in 2022, at 12.9% higher than expected. This compares to 11.7% excess mortality for Australia. 
  • Excess mortality in 2023 was 5.2% higher than expected. The number of excess deaths decreased by almost 60% when compared to 2022. 
  • There have been several prolonged periods where mortality in major cities has been higher than the expected range. These include December 2021 to September 2022, November 2022 to March 2023, May to June 2023 and November to December 2023.
  • Mortality was 2.5% lower than expected in 2020, with prolonged decreases occurring between June and October of that year.
  • COVID-19 associated mortality as recorded on death certificates has been a significant contributor to excess mortality in major cities each year. The virus accounted for just over two-thirds of excess mortality in 2022 and 2023. 
Excess mortality by year, Major cities, 2020-23
2020202120222023
Expected110,920110,116111,382112,887
Observed108,162112,746125,718118,813
Excess-2,7582,63014,3365,926
% Excess-2.52.412.95.2
Reported deaths from or with COVID-198691,3469,6404,089

a. Data is provisional and subject to change.
b. Years are based on a sum of International Organization for Standardisation (ISO) weeks. Weeks start on Monday, and week 1 always contains the 4th of January. There are 53 weeks in 2020. There are 52 weeks in 2021, 2022 and 2023. 
c. Reported deaths 'from' or 'with' COVID-19 are as recorded on the death certificate. 
d. Includes all deaths (both doctor and coroner certified) that occurred by 31 December 2023 and were registered and received by the ABS by 31 October 2024.

a. Data is provisional and will change as additional death registrations are received.
b. The baseline period generating the regression model is from 2013 to 2019. The prediction window begins in 2020. 
c. Includes all deaths (both doctor and coroner certified) that occurred by 31 December 2023 and were registered and received by the ABS by 31 October 2024. 

Weekly all-cause mortality: Inner regional

  • Excess mortality for inner regional areas was highest in 2022 when mortality was 8.6% higher than expected. This is lower than the national excess mortality estimate (11.7%). 
  • In 2023 excess mortality was estimated at 3.6% above expected. The number of excess deaths decreased by almost 60% when compared to 2022. 
  • There have been several prolonged periods where mortality in inner regional Australia has been higher than the expected range, though these have been fewer and of shorter duration than those in major cities. These include January to March 2022, May to August 2022, and November 2022 to January 2023.
  • Mortality was mostly lower than expected in inner regional areas in both 2020 and 2021. 
  • COVID-19 associated mortality as recorded on death certificates has been a significant contributor to excess mortality in inner regional areas since 2022. The virus accounted for over 70% of excess mortality in 2022 and over 90% in 2023. 
  • Peaks in excess mortality corresponded with periods where higher numbers of deaths associated with COVID-19 were occurring in inner regional areas. 
Excess mortality by year, Inner regional, 2020-23
2020202120222023
Expected38,76538,94039,75340,363
Observed37,07038,84243,16541,804
Excess-1,695-983,4121,441
% Excess-4.4-0.38.63.6
Reported deaths from or with COVID-1923802,5181,346

a. Data is provisional and subject to change.
b. Years are based on a sum of International Organization for Standardisation (ISO) weeks. Weeks start on Monday, and week 1 always contains the 4th of January. There are 53 weeks in 2020. There are 52 weeks in 2021, 2022 and 2023. 
c. Reported deaths 'from' or 'with' COVID-19 are as recorded on the death certificate. 
d. Includes all deaths (both doctor and coroner certified) that occurred by 31 December 2023 and were registered and received by the ABS by 31 October 2024.

a. Data is provisional and will change as additional death registrations are received.
b. The baseline period generating the regression model is from 2013 to 2019. The prediction window begins in 2020. 
c. Includes all deaths (both doctor and coroner certified) that occurred by 31 December 2023 and were registered and received by the ABS by 31 October 2024. 

Weekly all-cause mortality: Outer regional

  • Outer regional Australia recorded the lowest excess mortality during the pandemic. The only year with excess mortality recorded was 2022, when excess mortality was estimated at 3.4% higher than expected. This was the lowest percentage of excess mortality recorded across all remoteness areas in that year.
  • There have been very few weeks where mortality in outer regional Australia has been higher than the expected range. This happened in February to March 2022 and June to August 2022.
  • COVID-19 associated mortality was the main contributor to excess mortality in 2022. In that year, excess mortality is estimated at 601 deaths for outer regional areas. In 2022 986 people living in outer regional areas died from or with COVID-19. 
Excess mortality by year, Outer regional, 2020-23
2020202120222023
Expected17,11117,20817,70018,157
Observed16,20416,91418,30117,965
Excess-907-294601-192
% Excess-5.3-1.73.4-1.1
Reported deaths from or with COVID-191820986548

a. Data is provisional and subject to change.
b. Years are based on a sum of International Organization for Standardisation (ISO) weeks. Weeks start on Monday, and week 1 always contains the 4th of January. There are 53 weeks in 2020. There are 52 weeks in 2021, 2022 and 2023. 
c. Reported deaths 'from' or 'with' COVID-19 are as recorded on the death certificate. 
d. Includes all deaths (both doctor and coroner certified) that occurred by 31 December 2023 and were registered and received by the ABS by 31 October 2024.

a. Data is provisional and will change as additional death registrations are received.
b. The baseline period generating the regression model is from 2013 to 2019. The prediction window begins in 2020. 
c. Includes all deaths (both doctor and coroner certified) that occurred by 31 December 2023 and were registered and received by the ABS by 31 October 2024. 

Weekly all-cause mortality: Remote

  • Due to the low number of weekly deaths in remote and very remote Australia, modelling by individual age groups was not possible (population is controlled for to enable comparisons). Predicting an expected number of deaths from a small base can lead to volatility. This is evident from the range of the expected numbers - the upper and lower bounds are wider to account for this high variation. 
  • Excess mortality was highest in 2022 at 11.5% higher than expected, similar to the 11.7% recorded nationally.  
  • Excess mortality was 4.2% higher than expected in 2023. The number of excess deaths decreased by over 60% when compared to 2022. 
  • For most of the pandemic, mortality in remote Australia has been in the expected range of variation.
  • COVID-19 associated mortality as recorded on death certificates has been a significant contributor to excess mortality in remote Australia since 2022. The virus accounted for approximately half of the excess mortality in 2022 and 80% in 2023. 
  • In 2020 and 2021 there were very low numbers of deaths associated with COVID-19 for those in outer regional areas. 
Excess mortality by year, Remote, 2020-23
2020202120222023
Expected1,8221,8091,8301,861
Observed1,8261,8932,0411,940
Excess48421179
% Excess0.24.611.54.2
Reported deaths from or with COVID-19npnp10664

a. Data is provisional and subject to change.
b. Years are based on a sum of International Organization for Standardisation (ISO) weeks. Weeks start on Monday, and week 1 always contains the 4th of January. There are 53 weeks in 2020. There are 52 weeks in 2021, 2022 and 2023. 
c. Reported deaths 'from' or 'with' COVID-19 are as recorded on the death certificate. 
d. Includes all deaths (both doctor and coroner certified) that occurred by 31 December 2023 and were registered and received by the ABS by 31 October 2024.

a. Data is provisional and will change as additional death registrations are received.
b. The baseline period generating the regression model is from 2013 to 2019. The prediction window begins in 2020. 
c. Includes all deaths (both doctor and coroner certified) that occurred by 31 December 2023 and were registered and received by the ABS by 31 October 2024. 

Weekly all-cause mortality: Very remote

  • Due to the low number of weekly deaths in remote and very remote Australia, modelling by individual age groups was not possible (population is controlled for to enable comparisons). Predicting an expected number of deaths from a small base can lead to volatility. This is evident from the range of the expected numbers - the upper and lower bounds are wider to account for this high variation. 
  • Very remote Australia recorded the highest proportion of excess mortality in 2022 at 13.4%. This decreased by just over 80% in 2023.
  • Excess mortality for very remote Australia in 2023 was estimated at 2.5% above expected, down from 13.4% in 2022.
  • For most of the pandemic, mortality in very remote Australia has been in the expected range of variation.
  • COVID-19 associated mortality (as recorded on death certificates) accounted for about 40% of excess mortality in 2022. This was a slightly lower proportion than recorded in other remoteness areas. The virus accounted for almost all excess mortality in 2023. 
  • No deaths were recorded on death certificates as being due to COVID-19 for people living in very remote areas in 2020. 
Excess mortality by year, Very remote, 2020-23
2020202120222023
Expected1,1141,0991,1161,137
Observed1,0661,1311,2651,165
Excess-483214928
% Excess-4.32.913.42.5
Reported deaths from or with COVID-190np6029

a. Data is provisional and subject to change.
b. Years are based on a sum of International Organization for Standardisation (ISO) weeks. Weeks start on Monday, and week 1 always contains the 4th of January. There are 53 weeks in 2020. There are 52 weeks in 2021, 2022 and 2023. 
c. Reported deaths 'from' or 'with' COVID-19 are as recorded on the death certificate. 
d. Includes all deaths (both doctor and coroner certified) that occurred by 31 December 2023 and were registered and received by the ABS by 31 October 2024.

a. Data is provisional and will change as additional death registrations are received.
b. The baseline period generating the regression model is from 2013 to 2019. The prediction window begins in 2020. 
c. Includes all deaths (both doctor and coroner certified) that occurred by 31 December 2023 and were registered and received by the ABS by 31 October 2024. 

Methodology

The analysis of 2020-2023 mortality data is based on a model developed by Serfling¹⁰ and later adapted by the US Center for Disease Control (CDC) and the Centre for Epidemiology and Evidence at New South Wales Ministry of Health Health (NSW Health). This section provides an overview of how the model has been developed over time, key aspects of the model, and how the model has been adapted and applied by the ABS in this analysis.

Historical development of model

Historically this type of model has been used to estimate excess mortality caused by influenza.

As early as 1932, Collins determined that excess mortality during winter months in the United States of America (USA) was a consequence of epidemic influenza and therefore could be used as an indicator for the recognition of influenza outbreaks.²

In 1963, Serfling described a cyclic regression model, based on the seasonal pattern of pneumonia and influenza (P&I) deaths, to infer excess deaths due to influenza.¹⁰

Since then, models based on Serfling's have been applied in a number of temperate countries (USA,³,⁴ France,⁴ Australia,⁴ and Italy⁵) to demonstrate that excess mortality occurring during winter months is associated with the pandemic and seasonal epidemics of influenza.

The CDC took the approach further and established the 122 Cities Surveillance System to provide timely, prospective information on excess mortality due to influenza. ⁶,⁷

The New South Wales mortality surveillance was included as part of the routine Influenza Surveillance Program prior to COVID-19 and used an adaptation of the Serfling model to monitor for influenza epidemics. This system primarily monitored the proportion of influenza positive specimens among all respiratory specimens from the major public health laboratories, the proportion of emergency department visits diagnosed with influenza, and outbreaks of influenza reported to Public Health Units by residential care facilities.⁹ 

Aspects of the Serfling model and its implementations

The Serfling model uses historical data to predict current patterns of mortality. This enables the identification of any deviation from that prediction that may signal an influenza epidemic. In developing this model, Serfling recognised that past influenza epidemics could overly influence the regression, limiting the ability of the model to identify any new epidemic. For this reason, Serfling excluded past epidemics when fitting the model. 

An important feature of this methodology is the identification of a threshold that can be used as the upper and/or lower limit for the data. Observations that lie between these upper and lower limits are considered to be normal. The chosen threshold will vary depending on the desired specificity of the analysis and the methods used for calculation. In statistics there are default or familiar thresholds used in measuring these boundaries such as 1.96 standard errors either side of the mid-point.

An upper threshold was chosen to define the upper limit of the expected number of deaths in the absence of an influenza epidemic. This upper threshold was calculated as the predicted number of deaths in a given week plus a constant multiple of the standard error of the time series (the differences between each value predicted by the model and the actual observed values, or the 'model residuals'). Serfling chose the constant multiple to be 1.64 standard errors, and considered 2 consecutive weeks above the threshold to indicate epidemic behaviour.¹⁰

The NSW Health model used for the Influenza Surveillance Program is based on the Serfling model. This variant of the model fits a cyclic linear regression to data for the previous 5 calendar years and then forecasts the current year's time series from data up to the end of the previous year.

Data source

Please see the methodology section of Provisional Mortality Statistics for a description of the scope of data used in this model.

ABS adaptation of the model

The ABS has applied key characteristics of the model used in the New South Wales Influenza Surveillance Program. The ABS model is also based on the application of a cyclic linear regression model to the time series of weekly age-specific death rates. The model has been applied to both doctor certified and coroner referred deaths from all causes. 

The approach used by the ABS involves use of a 'robust' estimation procedure for fitting the model. Epidemics can cause increased mortality which leads to outliers in time series data. By down-weighting outliers it aims to fit the model to patterns of mortality that are expected to occur in the absence of an epidemic. This robust regression down-weights the influence of extreme observations (outliers) and is applied to a seven-year baseline time series (weeks commencing on a Monday). Some additional adjustments were made to extreme outliers, for example the 2017 influenza season. This adjustment involved applying a missing value to some age groups across some weeks where the actual rate was unusually high. This was done separately for each jurisdiction. 

The cyclical regression model includes: a linear time term, t, with values 1, 2, 3, ... for each week of the time series. Also included are annual seasonal harmonic variables to describe the cyclical seasonal background pattern. The harmonic variables are functions of the week number, t, and the periodicity in the same units – in this case, yearly (52 weeks). The 2 harmonic variables in this case are: sine(2π t/52) and cosine(2π t/52). For years with 53 weeks these harmonic terms have been divided by 53 instead of 52. The harmonic terms reflect the fact that the number of deaths is usually higher in winter than in summer.

The final model was:

\(Expected(proportion)=A+Bt+CSin(2πt/52)+DCos(2πt/52)\)
 

where A, B, C and D, are the coefficients calculated from the regression. Previous versions of this model also include a time squared term. This term was removed as it resulted in increasingly unrealistic output as the prediction window extended further from the modelling window.

To evaluate the approach, the model was fitted using PROC ROBUSTREG in SAS Software with the simplest, default 'M estimation' method.¹² The PROC SCORE procedure was then used to forecast values for 2020-2023. The standard error and threshold is derived from the stdi option in PROC ROBUSTREG which is run a second time with the 2020-2023 results attached.

Threshold identification

Identifying the threshold at which to signal excess mortality will vary depending on the goals of the research, application of the method, and seasonality of diseases of interest. Several national statistical organisations are using different models including Farrington surveillance algorithms, z -scores and prediction intervals to inform such thresholds.

A method for calculating the threshold for this analysis was selected which had a broader application and was more easily interpreted across a range of conditions, namely 1.96 standard errors or a 95% confidence interval. Use of the 95% confidence interval does introduce the expectation that random weekly variation could lead to counts that exceed thresholds 5% of the time. As such, individual weeks that exceed thresholds should be interpreted with caution. However, any prolonged period of weeks, 2 or more, exceeding the expected range can suggest excess mortality has been signalled.

The SAS function PROC ROBUSTREG offers the STDI parameter to calculate the 'standard error of prediction' of the model¹² which is used to define 1 standard error. The standard error of prediction is a more logical choice for assigning the threshold of excess mortality than the root mean square error (standard error of the model residuals) because it incorporates not only the variance of the residuals but also the variance of the model parameter estimates. This provides an estimate of the expected variability of the observed values in the absence of an epidemic.

Determining the baseline

Choosing the reference period (i.e. the number of years in the baseline) is important as it can change the expected number of deaths. When selecting the reference period key attributes were required:

  • There needed to be enough input available to predict the number of deaths from 2020-2023. As the paper is looking at excess mortality in the absence of the pandemic, no data from 2020-2023 is part of the reference period when predicting the expected number of deaths. Estimates of excess mortality are more accurate for years that are closer to the reference period.
  • A stable and clear mortality trend needed to be identified. This was particularly important as the model used is a harmonic with trend, meaning the pattern of death should be stable over a period to estimate an expected trend accurately.
  • The baseline period needed to be applied consistently across jurisdictions.

To make the decision, a sensitivity analysis was conducted. This analysis tested three reference periods: 2010-2019, 2013-2019 and 2015-2019. Standardised death rates were used to determine which years to test (these are presented below). Each baseline produced slightly different expected counts of deaths, which altered the % excess figures. The ABS decided to implement 2013-2019 as the predictor reference period. This baseline was chosen because:

  • There was a large decline in mortality between 2017 and 2018. This is likely due to the severe influenza season in 2017 causing some mortality displacement in 2018. Mortality displacement is an epidemiological concept which describes the phenomenon of a period of very high mortality being followed by a period of low mortality. Even controlling for 2017, the model was overcompensating for the rate of decline during 2015-2019, resulting in a very low number of expected deaths in 2022 and 2023.
  • Not all jurisdictions experienced a severe influenza season in 2017. Western Australia for example, had higher mortality rates in 2015 and 2016. Variability across jurisdictions meant that 2015-2019 had different outcomes across jurisdictions.
  • Western Australia had steeper declines in mortality rates between 2015-2016 and 2017 compared to other jurisdictions. Starting the baseline at the highest mortality points was overstating the rate of decline in WA and resulting in a very low number of expected deaths in 2022 and 2023. This was also affected by the model – as a harmonic with trend model is being used, volatility in the input trend affected the expected number of deaths.
  • For smaller jurisdictions with low numbers of weekly deaths, adding additional years onto the baseline provided a more stable trend.
  • There was some excess mortality in 2014, 2015 and 2017. Adjustments were made to outliers to control for these. 2013 was a year of stable mortality where no adjustments had to be made across any jurisdictions.
  • 2010-2019 and 2013-2019 produced similar results. However, there is less population change to account for from 2013 onwards. This was especially important for smaller jurisdictions where age adjustment was not as precise due to small numbers.
  • This model will also be used for analysis of diseases. There were a number of coding changes in 2013. Starting the reference period ensures continuity of time series for this analysis.

Modelling rates

Weekly mortality rates were calculated for 2013-2019 to predict expected weekly rates of death for 2020-2023. Rates were then converted to numbers for ease of interpretation.

Age groups selected for modelling ASDRs went through a number of assessments including:

  • A consistent number of deaths each week, ideally at least 20 deaths per week in the age group.
  • Population to be used as denominators should have at least 30 people in each age group.

Final age groups vary by jurisdiction based on the above criteria. Where possible, age groups were modelled separately up to 95 years and over (Australia, New South Wales, Victoria and Queensland). For South Australia and Western Australia age groups were modelled up to 90 years and over. In Tasmania age groups were modelled to 85 years and over. Weekly numbers of deaths for the Northern Territory and the Australian Capital Territory were too small to allow robust age-specific rates to be calculated, and crude rates (total population) were used instead.

Consideration was given to whether age-specific rates or age-standardised rates should be modelled. Both age-specific rates and age-standardised rates control for the age composition of a population and take into account growth rates within a population over time. Age-specific rates show the intensity of deaths within a population and express real-life mortality and population loss. Age-standardised rates are a modelled rate, standardised against a hypothetical population to enable comparison across cohorts. As comparison was not the main focus, age-specific rates were chosen as the model input.

Adjustment to Victorian death data

Every week there is typically a small number of death records where it is not possible to assign a remoteness area. This may be due to the death of a person who usually lived overseas, or inadequate detail of the usual residence of the deceased. During 2018 the Victorian Registry of Births, Deaths and Marriages implemented new software that resulted in several weeks of data where a higher than usual number of records did not have sufficient address details supplied to the ABS to allocate these records to a remoteness area. For this publication, 551 records for deaths that occurred in Victoria within week 18 and week 25 of 2018 have had their remoteness area imputed based on the average distribution by remoteness area by age for Victoria for the 10 weeks beginning in week 27 of 2018.

References

1. CDC. Excess Deaths Associated with COVID-19 https://www.cdc.gov/nchs/nvss/vsrr/covid19/excess_deaths.htm#techNotes

2. Collins SD. Excess mortality from causes other than influenza and pneumonia during influenza epidemics. Public Health Rep 1932;47:2159–2180.

3. Reichert T, Simonsen L, Sharma A, Pardo S, Fedson D, Miller M. Influenza and the winter increase in mortality in the United States, 1959–1999. Am J Epidemiol 2004;160:492–502.

4. Viboud C, Boelle P, Pakdaman K, Carrat F, Valleron A, Flahault A. Influenza epidemics in the United States, France, and Australia, 1972–1997. Emerg Infect Dis 2004;10:32–39.

5. Rizzo C, Bella A, Viboud C, Simonsen L, Miller M, Rota M, et al. Trends for influenza-related deaths during pandemic and epidemic seasons, Italy, 1969–2001. Emerg Infect Dis 2007;5:694–699.

6. Centers for Disease Control and Prevention. 122 Cities Mortality Reporting System, Manual of Procedures. Atlanta (Georgia, United States): US Department of Health and Human Services; 2004.

7. Simonsen L, Clarke MJ, Stroup DF, Williamson GD, Arden NH, Cox NJ. A method for timely assessment of influenza-associated mortality in the United States. Epidemiology 1997;8:90–395.

8. O'Brien K, Barr IG. Annual Report of the National Influenza Surveillance Scheme, 2006. Commun Dis Intell 2007;31:167–179.

9. NSW Health, Communicable Diseases Branch. New South Wales Influenza Surveillance Report. NSW Health.

10. Serfling RE. Methods for current statistical analysis of excess pneumonia-influenza deaths. Public Health Rep 1963;78:494–506.

11. The ROBUSTREG procedure. SAS/STAT(R) 12.3 User's Guide. Cary (USA): SAS Institute, 2013. Available from: https://support.sas.com/documentation/onlinedoc/stat/123/rreg.pdf

12. Model Fit and Diagnostic Statistics. SAS/STAT(R) 9.2 User's Guide. Cary (USA): SAS Institute, 2008.

Data downloads

Excess mortality, Remoteness areas, Jan 2013 - Dec 2023

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