Published: 06-02-2024 12:42 | Updated: 07-02-2024 08:32

Factors other than weaker variants behind reduced mortality in COVID-19

The image shows illustrated and colourful coronavirus with a microscope in the background.
Illustration: Gerd Altmann/Public Domain

Researchers at Karolinska Institutet, together with partners in the Horizon Europe-funded EuCARE project, have shown that the reduced mortality from COVID-19 is not necessarily due to the fact that later variants, such as Omicron, have been less severe. Rather, the reduced mortality seems to be due to several other factors, such as immunity from previous vaccinations and previous infections. The study is published in the latest issue of Lancet Regional Health Europe and is funded by EUH2020.

Portrait of a man wearing a shirt and jacket.
Pontus Hedberg, first author of the study.

The researchers conducted a study using patient data from more than 38,500 hospitalised patients with COVID-19, from the start of the pandemic to October 2022. The data comes from hospitals in ten countries, including two outside Europe.

The data showed that in-hospital mortality decreased as the pandemic progressed, especially since Omicron became the dominant variant. However, when the researchers modelled the mortality rates for different variants (Pre-Alpha, Alpha, Delta and Omikron) and took into account factors such as age, gender, comorbidity, vaccination status and time period, they saw far fewer differences and weaker associations. They also saw differences between age groups, highlighting the importance of conducting separate analyses for different age groups. 

“Overall, our findings suggest that the observed reduction in mortality during the pandemic is due to multiple factors such as immunity from vaccination and previous infections, and not necessarily tangible differences in inherent severity,” says Pontus Hedberg, first author of the study. 

Omicron variant no less severe 

Understanding the disease course and outcomes of patients hospitalised with COVID-19 during the pandemic is important to guide clinical practice and to understand and plan future resource use for COVID-19. A particularly interesting finding is that the inherent severity of Omicron has not necessarily been significantly reduced, but that other factors are behind the reduction in mortality. 

“The fact that Omicron can cause severe disease was seen in Hong Kong, for example, where the population had low immunity from previous infections and low vaccination coverage. In Hong Kong there was a relatively high mortality from Omicron," says Pontus Hedberg. 

Highlights the importance of protecting the elderly and those with underlying diseases

The main applications of the study results going forward are the continued need to protect the elderly and patients with other underlying disease from severe disease outcomes through vaccination against COVID-19, even though new virus variants may appear less virulent. The results are also important for understanding trends in mortality in hospitalised patients with COVID-19 and thus planning for resource use in hospital care.

Larger multinational collaborative projects like this are of great value to increase the generalisability of studies and not least to promote international collaboration also for future pandemic or epidemic scenarios.

This research received the contribution of the EuCARE Project funded by the European Union´s Horizon Europe Research and Innovation Programme under Grant Agreement No 101046016.

Anders Sönnerborg received grants for institutions from Gilead Sciences and GSK and consulting fees and speaker’s honoraria from AstraZeneca, Moderna, GSK, Gilead Sciences, and MSD, all out of the present work. Björn-Erik Ole Jensen received consulting fees and speaker’s honoraria from GSK, ViiV Healthcare, Gilead Sciences, MSD, Pfizer, AstraZeneca, Janssen-Cilag, Fresenius Medical Care and Falk Foundation, as well as support for attending meetings and travel from Gilead Sciences, all out of the present work. Milosz Parczewski received honoraria for presentation from Gilead Sciences, Abbvie, MSD, Janssen-Cilag, Roche, GSK, Virology Education, and Pfizer. Maurizio Zazzi received grants for institutions from Gilead Sciences, MSD, Theratechnologies and ViiV Healthcare and consulting fees and speaker’s honoraria from GSK, Gilead Sciences, MSD and ViiV Healthcare, all out of the present work. All other authors reported no conflicts of interest.