Researchers estimate COVID-19 Omicron variant mortality in Denmark

By Dr. Priyom Bose, Ph.D.Jan 23 2023Reviewed by Aimee Molineux

In Denmark, all forms of social restrictions that were implemented to restrict the spread of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the causal agent of the coronavirus disease 2019 (COVID-19) pandemic, were lifted in early February 2022.

Subsequently, a rapid increase in COVID-19 cases was recorded, predominantly caused by the Omicron variant. 

Background

Scientists have stated that the Omicron variant contains many genomic mutations regarding the original SARS-CoV-2 strain. This newly emerged variant is more infectious than the ancestral strain and can evade immune responses generated via natural infection or COVID-19 vaccination. However, the Omicron variant is less virulent than the previously emerged SARS-CoV-2 variants, e.g., the Delta strain.

Denmark’s policy to lift all restrictions amidst the high prevalence of COVID-19 cases gained significant international attention, particularly in the context of a steep rise in mortality rate due to COVID-19. This mortality report was based on a 30-day COVID-19 death count.

The main advantage of including a 30-day COVID-19 death count in routine reporting is its low resource requirement. Furthermore, this data is easily available in public databases and dashboards and is also used by international COVID-19 monitoring sites. Hence, the 30-day COVID-19 death count has been considered an important tool for evaluating COVID-19 severity during the earlier phases of the pandemic.

It is not easy to determine mortality due to COVID-19, as pinpointing the specific cause of death, particularly in the elderly population, is difficult. This is because, even if the deceased individual had contracted COVID-19, other underlying conditions could have been the cause of death. In addition, COVID-19 deaths in individuals who were not tested will not be captured in the 30-day COVID-19 death count.

However, because of the high rate of SARS-CoV-2 testing in Denmark, the possibility of additional deaths due to COVID-19 should be a rare incident. Nevertheless, it is imperative to develop a new methodology to quantify the real burden of COVID-19 mortality.

Mortality surveillance is extremely complex and vulnerable to bias and other limitations, which can be rectified by introducing different strategies in the surveillance. The Danish surveillance system offers three data sources linked to the mortality burden of COVID-19. These include individual death certificates, automated register-based surveillance, and deviations in all-cause mortality patterns.

About the study

A recent Eurosurveillance study investigated how coincidental infections influenced COVID-19 mortality estimates after the emergence of the Omicron variant. 

The current study analyzed the actual mortality burden of the Omicron variant by comparing the 30-day COVID-19 death count with the observed mortality pattern, using three surveillance methods, i.e., the Causes of Death Registry, a model created to correct the 30-day COVID-19 death count for coincidental deaths, and all-cause excess mortality.

Since mortality and SARS-CoV-2 infection are age dependent, the current estimations were carried out for different age groups, i.e., 0–19, 20–39, 40–59, 60–69, 70–79, and ≥ 80 years.

Findings

In Denmark, the total number of 30-day COVID-19 case fatalities from the beginning of the pandemic to 29 May 2022 was estimated to be 6,363. The daily number of deaths fluctuated significantly during the different COVID-19 waves. 

A retrospective comparative analysis revealed that the mortality burden due to COVID-19 weakened after introducing the Omicron variant in Denmark. Nevertheless, the mortality estimates projected in the international media suggested otherwise. Among all the waves that occurred due to the emergence of different SARS-CoV-2 variants, the maximum number of COVID-19 cases was observed in late 2021 due to the Omicron variant infection.

Although in most phases of the pandemic, the 30-day COVID-19 case incidence was low, a steep rise in cases was reported during the winter of 2021-22 because of the high transmission rate of the circulating Omicron variant and an increase in COVID-19 testing rates. The comparative analysis revealed a significant change in the share of coincidental deaths, which increased from 10–20% to around 40% after the emergence of the Omicron variant.

Even though a brief increase in the mortality rate was observed after the lifting of all COVID-19-related social restrictions in Denmark, the mortality rate became stable at a low-to-moderate level during the last quarter of 2021.

Conclusions

The current study confirms an important shift in COVID-19 mortality after the emergence of the Omicron variant, i.e., attenuation of mortality rates. The limitations associated with over/under-estimating mortality rates due to COVID-19 could be overcome by the new retrospective in-depth analysis method, which involves adjusting COVID-19 deaths for coincidental deaths.