Mortality is declining in Denmark

In the final months of 2021, Denmark witnessed an excess mortality among elderly people aged 75 years or more. In particular, from Week 49 in 2021 to Week 1 in 2022, the excess mortality was considerable and presumably caused by transmission of infection with the Delta variant.

More people have died “with” COVID-19 after the spread of the Omicron variant

COVID-19 transmission and morbidity have changed following the spreading of the Omicron variant in Denmark in late 2021. As the population sees a rise in the number of cases, many of which are less serious, the share of deaths incorrectly recorded as having occurred “due to” COVID-19 rather than “with” COVID-19 is expected to rise.

Therefore, the daily COVID-19 mortality rate will likely become less accurate than has been the case so far. Even so, the mortality rate may be used to assess mortality, because the majority of those affected will still die “due to” COVID-19.

How COVID-19-related deaths are registered

The continuous monitoring of COVID-19-related deaths is 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. Deaths are monitored in real time as recommended by the WHO and by recording excess mortality.

In Denmark, all deaths are registered on a Cause of Death Certificate, based on specific medical assessment of the cause of death. Even so, the figures carry some uncertainty, as it is often difficult to state the exact cause of death, and several competing causes are often in play; particularly if the person in question is affected by one or more underlying conditions. Therefore, the notification of the primary and any contributory causes of death is typically based, in part, on the physician’s estimation.

The completed Cause of Death Certificates are sent to the Danish Health Authority (DHA) for validation, after which each death is registered in the Danish Cause of Death Register. The process typically takes 1-2 weeks, but may in some cases take longer to complete.

Method recommended by the WHO

To achieve near-real-time monitoring of COVID-19-related deaths, the daily number of deaths is registered in the Central Person Register (CPR) in conjunction with COVID-19 test results. By definition, all daily 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. This definition is recommended by the WHO and is the standard definition used in various countries. Furthermore, the definition is used when reporting influenza-related deaths.

The advantage of using this 30-day COVID-19 mortality rate is that the rate may be extracted automatically from the national registers and can therefore be presented in real time. The drawback of the figure is that the method used will, in some cases, misclassify a death as a COVID-19 death even though the person died due to other causes, e.g., if a person dies due to a traffic accident after having become infected with COVID-19 and therefore did not die “due to” COVID-19 but rather “with” COVID-19. Therefore, the number of COVID-19 deaths is overestimated by the recorded numbers. The PandemiX Center at Roskilde University Copenhagen has estimated that up to 40% of the covid-19 related deaths in week 4 may be just coincidentally associated with a positive SARS-CoV-2 test. On the other hand, a slight underestimation of the number of COVID-19 deaths also occurs because any deaths occurring more than 30 days after a positive test are not part of this statistic.

Deaths with several underlying conditions

The mortality statement is further complicated by the fact that the overwhelming majority of deaths occur in persons who have underlying diseases. In these cases, COVID-19 will often have played an important part in the death, but the relative importance of COVID-19 and the other (underlying) diseases is difficult to assess. Therefore, the daily 30-day COVID-19 mortality rate is a trade-off, not completely accurate, but available in near-real-time. That is the reason why this mortality rate has been decisive in achieving near-real-time assessment of the development of the pandemic since the pandemic started.

New figures in the pipeline

To clarify and extend our understanding of COVID-19 mortality, Statens Serum Institut (SSI) in collaboration with the Danish Health Data Authority are currently exploring how the COVID-19 mortality figures from the Causes of Death Registry may be used to supplement the daily 30-day COVID-19 mortality rates, based on analyses of relevant diagnosis codes, even though this will introduce some delay. The SSI expects to be able to present these supplementary mortality rates soon, possibly stated on a weekly bases, once the necessary systems and data usage agreements are in place.

Excess mortality is an important signal when assessing the severity of the COVID-19 pandemic

Even though the number of COVID-19-related deaths is climbing, mortality is, in fact, declining at the moment. Therefore, it is important to take into consideration data describing excess mortality. The SSI analyses total excess mortality in the population in a manner that is independent of testing activity, diagnosis codes, etc. The SSI operates the EuroMOMO programme, which presents the observed total mortality by comparing it to the expected mortality, in Denmark and in 26 other European countries.

The Omicron variant has been dominant since mid-December, and the incidence among age groups above 65 years has followed an increasing trend since Week 51. Even so, the increasing transmission among elderly people does not seem to be accompanied by a corresponding excess mortality.

For more information about monitoring of the mortality in Denmark, please see the SSI’s website (in danish)

You may also read more about monitoring of the mortality in Europe at the website of EuroMOMO

Figure 1. Overall mortality in Denmark per week in the course of 2021