New research on excess mortality in Nordics during COVID

From a scientific and policy perspective, it is important to record and examine the additional deaths during the coronavirus disease 2019 (COVID-19) pandemic.

Excess mortality refers to the number of all-cause deaths that occur during a crisis beyond the number expected under ‘normal’ conditions. It is vitally important that we understand how the death toll during the COVID-19 pandemic compares to what we would expect had it not occurred – a crucial amount that cannot be known but can be estimated in a number of ways. Compared with the confirmed COVID-19 death count alone, excess mortality provides a more comprehensive measure of the impact of the pandemic on deaths. It includes not only confirmed deaths, but also deaths related to COVID-19 that have not been properly diagnosed and reported. Also includes deaths from causes other than COVID-19 related to crisis conditions.

Study: Excess mortality estimates for the five Nordic countries during the 2020-2021 Covid-19 pandemic. Image credit: NIAID

The study

A recent study published in medRxiv* The preprint server reviewed the methods used to estimate final all-cause deaths and the uncertainties in these methods for the five Nordic countries (Sweden, Norway, Denmark, Iceland and Finland).

Nordic countries were selected because they are historically and culturally interconnected; they maintain high quality public health data; an extensive survey was conducted in all countries during the study period, and final annual all-cause mortality rates for 2020 and 2021 were available.

The latest high-quality record was used to assess death estimates using linear interpolation. In addition, the required expected deaths have been back-calculated from annual Nordic all-cause death data so that reported excess deaths appear accurate.

The researchers conducted this study to critically review existing methods for recording deaths and estimate uncertainties, implications, plausible ranges, and limitations in the current debate about per capita deaths and recording differences.

All-cause deaths in the Nordic countries 2010-2021 (squares).  (A) Denmark.  (B) Finland.  (D) Iceland.  (D) Norway.  (I) Sweden.  The red lines show the backward calculated expected deaths (2020 and 2021 average) implied by the excess deaths in Wang et al.

All-cause deaths in the Nordic countries 2010-2021 (squares). (A) Denmark. (B) Finland. (D) Iceland. (D) Norway. (I) Sweden. The red lines show the backward calculated expected deaths (2020 and 2021 average) implied by the excess deaths in Wang et al.

discoveries

One of these methods (by Wang et al.) showed results that were different from all other estimates included in this study. Therefore, an additional analysis was performed for this specific estimate.

Reverse calculation of estimated deaths revealed that the numbers by Wang et al. did not match the actual data. Consequently, the excess deaths may be overestimated compared to reasonable variations in data from Finland, Denmark and, to some extent, Sweden.

The main uncertainties in the record of excess deaths were the 2018 flu – more for Denmark and to a lesser extent for Finland, and Sweden’s low mortality in the year 2019.

After reviewing estimation methods and sensitivity tests, it was determined that the overall excess deaths in these five Nordic countries were between 15,000 and 20,000. The number suggested by the World Health Organization (WHO) published shortly before this article was 17,716.

Barber et al.12 implicit infection mortality rates using methodology similar to Wang et al.9 for deaths and corresponding numbers for other methods obtained using the scale factors in Table 1.(* Economist model excluding January to February 2020 from trend calculation).

Implicit infection mortality rates from Barber et al. using methodology similar to that of Wang et al. for deaths, and corresponding numbers for other methods obtained using the scale factors in Table 1

However, these results were about half of those proposed by Wang et al. and emphasize that these countries had a similar ability to record COVID-19 related deaths. In addition, infection-related mortality rates matched pandemic management expectations and were also more homogeneous.

Finland and Denmark revealed heterogeneous results, with significantly lower ability to identify deaths related to COVID-19 and extraordinarily high severity of infection. The Wang et al. model implied this and these conclusions were made, due to the very high death rates for these countries.

Importantly, excess total numbers cannot directly affect performance estimates nor policy implications, even if they were accurate, as they do not confound population age changes over time.

Inference

According to the review, a method for estimating deaths where hard data is not yet available is likely to fail substantially when hard data are available, which may have implications for other countries and global estimates. The study illustrates the need for quality control of complex models that may contain uncertainties and assumptions that may be difficult to interpret in general. The message must be clear for policy implications and the general public, but high-quality data must not be overshadowed by complex models.

*Important news

medRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered conclusive, guide clinical practice/health-related behavior, or treated as established information.

New research on excess mortality in Nordics during COVID

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