Excess Mortality: Considerations in Moving Away from a Pre-pandemic Baseline
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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'?"
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.
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.
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:
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.
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?
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.
The table shows that:
In our view, a reasonable estimate of a baseline for 2024 would:
We have applied this approach to the available data, noting that the figures shown here are indicative only; we have used simplified modelling techniques to illustrate the concept [3] .
The following graphs show for each non-respiratory, non-COVID-19 cause of death:
The following graphs show the same information for non-COVID-19 respiratory deaths and deaths from COVID-19. For the 2024 baseline, no mortality trend is assumed for respiratory deaths. For COVID-19 deaths, the 2024 baseline assumes two COVID-19 waves, each resulting in 15% less deaths than the previous wave (noting that the assumption of a 15% reduction per wave is indicative and our final assumption may differ from this).
The following graph shows the resulting all-cause SDRs.
This indicative modelling results in an all-cause baseline SDR for 2024 that is similar to the actual SDRs for 2020 and 2021.
The excess mortality question posed by the suggested baseline may be:
"How does 2024 mortality compare to the expected level based on 2023, after making allowance for reasonable expectations for mortality improvement?"
Or, more simply: "Is 2023 mortality the new normal?"
We acknowledge that:
We stress the high uncertainty (and wide confidence interval) relating to the resultant calculation of excess mortality.
While it is early to be thinking about a baseline for 2025 and later years, we wanted to make it clear that our adopted framework for the 2024 baseline applies to that year only.
We envisage that, as time progresses, the baseline would evolve and more post-2022 years would be included in setting the baseline.
Our current thinking is that:
The implementation of this idea will need further thought and development. It is reasonable to suppose that elements will change, depending on the actual mortality experience that emerges in the next couple of years.
The members of the Working Group are:
[1] https://www.ons.gov.uk/peoplepopulationandcommunity/healthandsocialcare/causesofdeath/articles/estimatingexcessdeathsintheukmethodologychanges/february2024
[2] Defined as where more than 15% of all deaths are from COVID-19
[3] Deaths from COVID-19 shown here include an estimate of coroner-referred deaths from COVID-19 (which are in turn deducted from the coroner-referred deaths).
