Mortality due to COVID-19 in Denmark is not increasing as much as are case numbers

Following the spreading of the Omicron variant in Denmark, more people become infected by COVID-19, but a more limited share than previously of those infected die due to their infection. This is what data from the Danish Causes of Death Registry show.

Statens Serum Institut continuously monitors COVID-19-related deaths based on the daily number of deaths registered by the Central Office of the Civil Registration (CPR) Registry in combination with COVID-19 test results.
By definition, all deaths for which a positive COVID-19 PCR test was recorded within the 30 days leading up to the date of death are counted as COVID-19 deaths. The same method is used for monitoring of other airway infections, e.g., influenza and pneumococci. Since the beginning of the epidemic, continuous monitoring has been an important element in the assessment of the prevalence and severity of the epidemic, along with COVID-19 case numbers and the number of people admitted to hospital with COVID-19.

“The daily surveillance figures from the SSI supplement the registration of all recorded deaths in the Causes of Death Registry. Even so, several causes often contribute to death, particularly if the deceased person already had one or more underlying diseases as is often the case among elderly people. Therefore, the stated primary and contributory causes of death typically reflect a measure of medical judgement on the part of the physician completing the death certificate,” notes Henrik Ullum, CEO at Statens Serum Institut.

The completed Cause of Death Certificates are sent to the Danish Health Data Authority (HDA) for validation, after which each death is registered in the Cause of Death Registry. The process typically takes a few weeks, but may in some cases take longer to complete. The HDA figures describing COVID-19 deaths are typically more accurate than the automated register data extractions, but, in contrast, these data are available almost immediately. To underpin the continuous monitoring and assessment of epidemic developments, 30-day COVID-19 deaths have been an important daily figure.

The Omicron variant has led to an increasing proportion of deaths “with” COVID-19 rather than deaths “due to” COVID-19

With the spreadof the Omicron variant and the advancement of booster vaccination efforts in Denmark as from late 2021, COVID-19 transmission and morbidity have changed character. As the population sees a rise in the number of cases, many of which are less serious, an increase is seen in the share of deaths incorrectly recorded as having occurred “due to” COVID-19 rather than “with” COVID-19. Therefore, the daily figures describing 30-day COVID-19 deaths have become less accurate than previously.

In collaboration with the HDA, the SSI has prepared a report describing the COVID-19 deaths recorded in the Causes of Death Registry based on death certificates carrying a range of COVID-19-related diagnosis codes.

The report confirms the pattern by showing that more people now die “with” COVID-19 than “due to” COVID-19 as high case numbers are being recorded due to the spreading of the Omicron variant. As shown in Figure 1, the share of all 30-day COVID-19 deaths assessed to have occurred “with” COVID-19 has followed an increasing trend since early 2022, whereas the number of deaths “due to” COVID-19 has remained largely stable. Even so, some uncertainty characterises the latest few weeks as a considerable share of death certificates has yet to be registered.

Figure 1. Deaths “due to” and “with” COVID-19 based on death certificates, from March 2020 to February 2022, compared with 30 COVID-19 deaths (red line). Reports from the Danish Causes of Death Registry, the Danish Health Data Authority and the SSI.

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Furthermore, the figure shows a generally high level of agreement between 30-day COVID-19 deaths and deaths recorded in the Causes of Death Registry in the period leading up to the spreading of the Omicron variant.

As COVID-19 transmission and morbidity have changed character following the spread of the Omicron variant and as vaccine-induced immunity has increased in Denmark since late 2021, it has become increasingly important to supplement the 30-day COVID-19 death figures by taking into account the reports from the Causes of Death Registry as these reports afford us an opportunity to assess more accurately how many deaths are, in fact, due to COVID-19.

The SSI will keep monitoring deaths related to the COVID-19 epidemic in Denmark by both systems.

For a more detailed memo on the SSI’s monitoring of mortality during the COVID-19 epidemic in Denmark, please see here