Post by Nadica (She/Her) on Jul 12, 2024 16:20:23 GMT
Excess Mortality: Considerations in Moving Away from a Pre-pandemic Baseline - Published April 24, 2024
Don't like those pesky excess death numbers from still-spreading covid? Just move the goalpost! Let's not even factor in that covid is causing many people to develop cardiac disease which has risen faster than other causes of death since 2020! I want a nicer number!
The Mortality Working Group is planning to make a significant change to the baseline (or expected number of deaths) used when measuring excess mortality for Australia.
The Working Group intends to move away from the philosophical question of “How has the pandemic affected mortality?” to instead answer the question “Is 2023 mortality the ‘new normal’?”
Purpose
The purpose of this article is to set out considerations for setting a baseline (or expected number of deaths) for use in excess mortality calculations for the year 2024 and beyond. Many of the finer details still need to be determined, but this article sets out the conceptual framework that the Mortality Working Group intends to adopt for measuring excess mortality in 2024.
Background
With the COVID-19 pandemic, there has been a huge amount of interest in excess mortality calculations – that is, estimates of the number of deaths over and above what would have been expected. To date, the bulk of these calculations, including ours, have used an expected number of deaths had the pandemic (COVID-19) not occurred. This has been an entirely appropriate way to estimate the baseline, and the resulting excess mortality is an estimate of the net mortality impact of the pandemic.
We are, however, now in the fifth year of COVID-19. This means that it is now several years since the population experienced pre-pandemic mortality. Extrapolations generally assume that pre-pandemic mortality trends would have continued, and it is debatable whether this is a valid assumption to hold for a five-year extrapolation period.
Consider, for example, the projected 2020 standardised death rate (SDR) based on linear trends of the five-year periods from 2011-2015 to 2015-2019. The projected 2020 SDR ranges from 504.3 (using 2015-19 as the base) to 534.6 (using 2013-17 as the base), a 6% difference, which is large in the context of measuring excess mortality.
It is also arguable that excess mortality calculations should no longer be trying to measure “the impact of the pandemic”. Alternative questions may be “How is current mortality tracking relative to pandemic years?” or “How is current mortality tracking relative to the last year?”
These questions may help determine whether we are now at a new normal level of mortality – i.e., is the 2023 mortality a reasonable starting point, or does it still include some pandemic-related “shocks” to mortality?
Clearly identifying what is meant by “excess mortality” will ultimately determine how to set the baseline.
New ONS methodology for calculating excess deaths
In February 2024, the UK’s Office for National Statistics released a methodology paper[1] setting out its new approach to measuring excess mortality. We are pleased to see that the ONS is moving to a methodology that allows for changes in both population size and age characteristics over time. The ONS has also decided to include pandemic years in the data used to set the baseline, however individual weeks and months with very high COVID-19 mortality[2] are removed.
This provides a communication challenge. While it is difficult to articulate, in our view the ONS has not clearly explained what the new baseline represents. Until now, most baselines (including those of the Mortality Working Group and the Australian Bureau of Statistics) have been expected deaths in the absence of the pandemic. The new ONS baseline is clearly not this, but what is it?
For 2024, the ONS baseline uses data from 2018-2023, covering:
two pre-pandemic years, one of which happened to have unusually low mortality; and
four pandemic years, excluding the two very high COVID-19 waves.
So, if 2024 results in no excess mortality, is this a good or a poor outcome?
We are not suggesting that this is a simple exercise but, at the same time, it seems somewhat unhelpful not to try to explain (in words) what the excess represents – the excess over and above what?
As time progresses, and the pandemic impact on mortality becomes a historical fact, this will become irrelevant. However, at the stage we are at now, having a mix of pre-pandemic and pandemic years used to determine the baseline is not easy to explain nor understand.
Estimate of including pandemic years for Australia
We have formed a fairly rough-and-ready estimate of the baseline for 2024 if we included pandemic years when setting the baseline. We have estimated the expected number of deaths in 2024 by fitting a linear trend to deaths from 2017 to 2023 (i.e., seven years of data). Our estimate here is very approximate and is provided as an indicative estimate only.
Using data to 31 December 2023, we estimate that total deaths in 2023 may be around 183,700 (after including a small allowance for late-reported deaths). If we include pandemic years in setting the 2024 baseline, the expected number of deaths in 2024 might be, roughly, around 190,000.
Based on what we know about the causes of mortality in 2022 and 2023, we think that this is an unrealistically high expected number of deaths. The mechanistic inclusion of data from pandemic years into the model does not produce a realistic estimate of the expected number of deaths. Even if some of the pandemic spikes in mortality were excluded, we expect that the application of the model would still produce an unrealistically high estimate of mortality for 2024.
As for the discussion of the ONS baseline, our key question is, what does the measured excess mortality mean? Would negative excess mortality in 2024 against the mechanistic seven-year trend convey the right message about 2024’s mortality?
What might a reasonable baseline for 2024 look like?
The following table is taken from our analysis of excess mortality to 31 December 2023. It shows the estimated excess broken down by cause of death for each of the pandemic years. Note that the excess is measured relative to pre-pandemic expectations of mortality and that these expectations build in the mortality trend for each cause.
Don't like those pesky excess death numbers from still-spreading covid? Just move the goalpost! Let's not even factor in that covid is causing many people to develop cardiac disease which has risen faster than other causes of death since 2020! I want a nicer number!
The Mortality Working Group is planning to make a significant change to the baseline (or expected number of deaths) used when measuring excess mortality for Australia.
The Working Group intends to move away from the philosophical question of “How has the pandemic affected mortality?” to instead answer the question “Is 2023 mortality the ‘new normal’?”
Purpose
The purpose of this article is to set out considerations for setting a baseline (or expected number of deaths) for use in excess mortality calculations for the year 2024 and beyond. Many of the finer details still need to be determined, but this article sets out the conceptual framework that the Mortality Working Group intends to adopt for measuring excess mortality in 2024.
Background
With the COVID-19 pandemic, there has been a huge amount of interest in excess mortality calculations – that is, estimates of the number of deaths over and above what would have been expected. To date, the bulk of these calculations, including ours, have used an expected number of deaths had the pandemic (COVID-19) not occurred. This has been an entirely appropriate way to estimate the baseline, and the resulting excess mortality is an estimate of the net mortality impact of the pandemic.
We are, however, now in the fifth year of COVID-19. This means that it is now several years since the population experienced pre-pandemic mortality. Extrapolations generally assume that pre-pandemic mortality trends would have continued, and it is debatable whether this is a valid assumption to hold for a five-year extrapolation period.
Consider, for example, the projected 2020 standardised death rate (SDR) based on linear trends of the five-year periods from 2011-2015 to 2015-2019. The projected 2020 SDR ranges from 504.3 (using 2015-19 as the base) to 534.6 (using 2013-17 as the base), a 6% difference, which is large in the context of measuring excess mortality.
It is also arguable that excess mortality calculations should no longer be trying to measure “the impact of the pandemic”. Alternative questions may be “How is current mortality tracking relative to pandemic years?” or “How is current mortality tracking relative to the last year?”
These questions may help determine whether we are now at a new normal level of mortality – i.e., is the 2023 mortality a reasonable starting point, or does it still include some pandemic-related “shocks” to mortality?
Clearly identifying what is meant by “excess mortality” will ultimately determine how to set the baseline.
New ONS methodology for calculating excess deaths
In February 2024, the UK’s Office for National Statistics released a methodology paper[1] setting out its new approach to measuring excess mortality. We are pleased to see that the ONS is moving to a methodology that allows for changes in both population size and age characteristics over time. The ONS has also decided to include pandemic years in the data used to set the baseline, however individual weeks and months with very high COVID-19 mortality[2] are removed.
This provides a communication challenge. While it is difficult to articulate, in our view the ONS has not clearly explained what the new baseline represents. Until now, most baselines (including those of the Mortality Working Group and the Australian Bureau of Statistics) have been expected deaths in the absence of the pandemic. The new ONS baseline is clearly not this, but what is it?
For 2024, the ONS baseline uses data from 2018-2023, covering:
two pre-pandemic years, one of which happened to have unusually low mortality; and
four pandemic years, excluding the two very high COVID-19 waves.
So, if 2024 results in no excess mortality, is this a good or a poor outcome?
We are not suggesting that this is a simple exercise but, at the same time, it seems somewhat unhelpful not to try to explain (in words) what the excess represents – the excess over and above what?
As time progresses, and the pandemic impact on mortality becomes a historical fact, this will become irrelevant. However, at the stage we are at now, having a mix of pre-pandemic and pandemic years used to determine the baseline is not easy to explain nor understand.
Estimate of including pandemic years for Australia
We have formed a fairly rough-and-ready estimate of the baseline for 2024 if we included pandemic years when setting the baseline. We have estimated the expected number of deaths in 2024 by fitting a linear trend to deaths from 2017 to 2023 (i.e., seven years of data). Our estimate here is very approximate and is provided as an indicative estimate only.
Using data to 31 December 2023, we estimate that total deaths in 2023 may be around 183,700 (after including a small allowance for late-reported deaths). If we include pandemic years in setting the 2024 baseline, the expected number of deaths in 2024 might be, roughly, around 190,000.
Based on what we know about the causes of mortality in 2022 and 2023, we think that this is an unrealistically high expected number of deaths. The mechanistic inclusion of data from pandemic years into the model does not produce a realistic estimate of the expected number of deaths. Even if some of the pandemic spikes in mortality were excluded, we expect that the application of the model would still produce an unrealistically high estimate of mortality for 2024.
As for the discussion of the ONS baseline, our key question is, what does the measured excess mortality mean? Would negative excess mortality in 2024 against the mechanistic seven-year trend convey the right message about 2024’s mortality?
What might a reasonable baseline for 2024 look like?
The following table is taken from our analysis of excess mortality to 31 December 2023. It shows the estimated excess broken down by cause of death for each of the pandemic years. Note that the excess is measured relative to pre-pandemic expectations of mortality and that these expectations build in the mortality trend for each cause.