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Reduced Risk of Hospitalisation Associated With Infection With SARS-CoV-2 Omicron Relative to Delta: A Danish Cohort Study

27 Pages Posted: 14 Jan 2022

See all articles by Peter Bager

Peter Bager

Statens Serum Institut - Department of Epidemiology Research

Jan Wohlfahrt

Statens Serum Institut

Samir Bhatt

Imperial College London - NIHR Health Protection Research Unit for Modelling and Health Economics

Sofie Marie Edslev

Statens Serum Institut

Raphael Niklaus Sieber

Statens Serum Institut

Anna Cäcilia Ingham

affiliation not provided to SSRN

Marc Stegger

Statens Serum Institut - Department of Bacteria, Parasites and Fungi

Rebecca Legarth

Statens Serum Institut

Camilla Holten Møller

Statens Serum Institut

Robert Leo Skov

Statens Serum Institut

Palle Valentiner-Branth

Statens Serum Institut - Infectious Disease Epidemiology & Prevention

Maria Overvad

affiliation not provided to SSRN

Mie Agermose Gram

Statens Serum Institut - Infectious Disease Epidemiology & Prevention

Frederikke Kristensen Lomholt

affiliation not provided to SSRN

Louise Hallundbæk

Statens Serum Institut

Caroline Hjorth Espensen

affiliation not provided to SSRN

Sophie Madeleine Gubbels

Statens Serum Institut - Department of Data Management and Analysis

Marianne Voldstedlund

affiliation not provided to SSRN

Marianne Karakis

affiliation not provided to SSRN

Karina Lauenborg Møller

affiliation not provided to SSRN

Stefan Schytte Olsen

affiliation not provided to SSRN

Thea K. Fischer

University of Copenhagen - Department of Public Health; University of Copenhagen - Department of Clinical Medicine

Zitta Barrella Harboe

University of Copenhagen - Department of Respiratory Medicine and Infectious Diseases

Caroline Klint Johannesen

Statens Serum Institut - Department of Virus and Microbiological Special Diagnostics

Maarten van Wijhe

Roskilde University - Department of Science and Environment

Jon Gitz Holler

affiliation not provided to SSRN

Lone Simonsen

Roskilde University - Department of Science and Environment

Ram Benny Christian Dessau

affiliation not provided to SSRN

Martin Barfred Friis

affiliation not provided to SSRN

David Fuglsang-Damgaard

affiliation not provided to SSRN

Mette Pinholt

affiliation not provided to SSRN

Nikolai Søren Kirkby

affiliation not provided to SSRN

Marianne Kragh Thomsen

affiliation not provided to SSRN

Thomas Vognbjerg Sydenham

affiliation not provided to SSRN

John Eugenio Coia

affiliation not provided to SSRN

Ea Sofie Marmolin

affiliation not provided to SSRN

Anders Fomsgaard

Statens Serum Institut

Jannik Fonager

Statens Serum Institut - Virus Research & Development Laboratory

Morten Rasmussen

Statens Serum Institut - Virus Research & Development Laboratory

Katja Spiess

Statens Serum Institut

Ellinor Marving

Statens Serum Institut

Arieh Cohen

affiliation not provided to SSRN

Nicolai Balle Larsen

affiliation not provided to SSRN

Troels Lillebaek

affiliation not provided to SSRN

Henrik Ullum

Statens Serum Institut

Kåre Mølbak

affiliation not provided to SSRN

Tyra Grove Krause

Statens Serum Institut

More...

Abstract

Background: Estimates of the severity of the emerging Omicron variant (lineage B.1.1.529) are critical to assess the public health impact associated with its rapid global dissemination. We estimated the risk of SARS-CoV-2-related hospitalizations after infection with Omicron compared to Delta overall and stratified by vaccination status in Denmark, a country with high mRNA vaccination coverage and an extensive free-of-charge PCR test capacity.

Methods: Observational cohort study of all RT-PCR SARS-CoV-2 positive individuals in Denmark between 21 November and 19 December, 2021, the period Omicron emerged while Delta was still the dominant variant. Individuals including hospitalised cases, were identified in the national COVID-19 surveillance system database including results of a variant specific RT-PCR detecting Omicron cases (vPCR), the National Patient Registry, the National Vaccination Registry, as well as other nationwide registers. The risk ratio (RR) of admission for Omicron compared to Delta infection, overall and stratified by vaccination status, was calculated in a Poisson regression model with robust standard errors, adjusted à priori for reinfection status, sex, age, region, comorbidities, and calendar week of sampling.

Findings: Among the 188,980 SARS-CoV-2 infected individuals, 38,669 (20.1%) were Omicron vPCR positive. SARS-CoV-2 related hospitalisations and Omicron cases increased during the study period. Overall, 65.8 % were vaccinated and vaccination was associated with a lower risk of hospitalisation (adjusted RR 0.25, 95%CI 0.22-0.27), compared to cases with no or one vaccination. Compared to Delta infection, Omicron was associated with an adjusted RR of hospitalisation of 0.64 (95%CI 0.56-0.75). The RR of hospitalisation was 0.57 (95%CI 0.44-0.75) among cases with no or one vaccination, 0.71 (95%CI 0.60-0.86) among two-dose vaccinated, and 0.50 (95%CI 0.32-0.76) among three-dose vaccinated.

Interpretation: We found a significantly lower risk of hospitalisation with Omicron infection compared to Delta among both vaccinated and unvaccinated individuals suggesting an inherent reduced severity of Omicron. Our results can guide modelling of the impact of the rapid spread of Omicron globally and thus health care system preparedness.

Funding Information: The authors received no specific funding for this work.

Declaration of Interests: We declare no competing interest.

Ethics Approval Statement: All analyses were based on information from the national COVID-19 surveillance system database updated and extracted on 4 January, 2022. This study was done with use of administrative register data. According to Danish law, ethics approval is not needed for such research.

Keywords: covid-19, SARS-CoV-2, virus, hospital admission, epidemiology

Suggested Citation

Bager, Peter and Wohlfahrt, Jan and Bhatt, Samir and Edslev, Sofie Marie and Sieber, Raphael Niklaus and Ingham, Anna Cäcilia and Stegger, Marc and Legarth, Rebecca and Holten Møller, Camilla and Skov, Robert Leo and Valentiner-Branth, Palle and Overvad, Maria and Gram, Mie Agermose and Lomholt, Frederikke Kristensen and Hallundbæk, Louise and Espensen, Caroline Hjorth and Gubbels, Sophie Madeleine and Voldstedlund, Marianne and Karakis, Marianne and Møller, Karina Lauenborg and Olsen, Stefan Schytte and Fischer, Thea K. and Barrella Harboe, Zitta and Johannesen, Caroline Klint and van Wijhe, Maarten and Holler, Jon Gitz and Simonsen, Lone and Dessau, Ram Benny Christian and Friis, Martin Barfred and Fuglsang-Damgaard, David and Pinholt, Mette and Kirkby, Nikolai Søren and Thomsen, Marianne Kragh and Sydenham, Thomas Vognbjerg and Coia, John Eugenio and Marmolin, Ea Sofie and Fomsgaard, Anders and Fonager, Jannik and Rasmussen, Morten and Spiess, Katja and Marving, Ellinor and Cohen, Arieh and Larsen, Nicolai Balle and Lillebaek, Troels and Ullum, Henrik and Mølbak, Kåre and Grove Krause, Tyra, Reduced Risk of Hospitalisation Associated With Infection With SARS-CoV-2 Omicron Relative to Delta: A Danish Cohort Study. Available at SSRN: https://ssrn.com/abstract=4008930 or http://dx.doi.org/10.2139/ssrn.4008930

Peter Bager (Contact Author)

Statens Serum Institut - Department of Epidemiology Research

Jan Wohlfahrt

Statens Serum Institut

Denmark

Samir Bhatt

Imperial College London - NIHR Health Protection Research Unit for Modelling and Health Economics ( email )

London
United Kingdom

Sofie Marie Edslev

Statens Serum Institut ( email )

Denmark

Raphael Niklaus Sieber

Statens Serum Institut ( email )

Denmark

Anna Cäcilia Ingham

affiliation not provided to SSRN ( email )

No Address Available

Marc Stegger

Statens Serum Institut - Department of Bacteria, Parasites and Fungi ( email )

Rebecca Legarth

Statens Serum Institut

Denmark

Camilla Holten Møller

Statens Serum Institut

Denmark

Robert Leo Skov

Statens Serum Institut

Denmark

Palle Valentiner-Branth

Statens Serum Institut - Infectious Disease Epidemiology & Prevention

Maria Overvad

affiliation not provided to SSRN ( email )

No Address Available

Mie Agermose Gram

Statens Serum Institut - Infectious Disease Epidemiology & Prevention

Frederikke Kristensen Lomholt

affiliation not provided to SSRN ( email )

No Address Available

Louise Hallundbæk

Statens Serum Institut ( email )

Denmark

Caroline Hjorth Espensen

affiliation not provided to SSRN ( email )

No Address Available

Sophie Madeleine Gubbels

Statens Serum Institut - Department of Data Management and Analysis ( email )

Copenhagen
Denmark

Marianne Voldstedlund

affiliation not provided to SSRN ( email )

No Address Available

Marianne Karakis

affiliation not provided to SSRN ( email )

No Address Available

Karina Lauenborg Møller

affiliation not provided to SSRN ( email )

No Address Available

Stefan Schytte Olsen

affiliation not provided to SSRN ( email )

No Address Available

Thea K. Fischer

University of Copenhagen - Department of Public Health ( email )

Denmark

University of Copenhagen - Department of Clinical Medicine ( email )

Nørregade 10
Copenhagen, København DK-1165
Denmark

Zitta Barrella Harboe

University of Copenhagen - Department of Respiratory Medicine and Infectious Diseases ( email )

Copenhagen
Denmark

Caroline Klint Johannesen

Statens Serum Institut - Department of Virus and Microbiological Special Diagnostics ( email )

Maarten Van Wijhe

Roskilde University - Department of Science and Environment ( email )

Roskilde
Denmark

Jon Gitz Holler

affiliation not provided to SSRN ( email )

No Address Available

Lone Simonsen

Roskilde University - Department of Science and Environment ( email )

Roskilde
Denmark

Ram Benny Christian Dessau

affiliation not provided to SSRN ( email )

No Address Available

Martin Barfred Friis

affiliation not provided to SSRN ( email )

No Address Available

David Fuglsang-Damgaard

affiliation not provided to SSRN ( email )

No Address Available

Mette Pinholt

affiliation not provided to SSRN ( email )

No Address Available

Nikolai Søren Kirkby

affiliation not provided to SSRN ( email )

No Address Available

Marianne Kragh Thomsen

affiliation not provided to SSRN ( email )

No Address Available

Thomas Vognbjerg Sydenham

affiliation not provided to SSRN ( email )

No Address Available

John Eugenio Coia

affiliation not provided to SSRN ( email )

No Address Available

Ea Sofie Marmolin

affiliation not provided to SSRN ( email )

No Address Available

Anders Fomsgaard

Statens Serum Institut

Denmark

Jannik Fonager

Statens Serum Institut - Virus Research & Development Laboratory ( email )

Copenhagen
Denmark

Morten Rasmussen

Statens Serum Institut - Virus Research & Development Laboratory ( email )

Copenhagen
Denmark

Katja Spiess

Statens Serum Institut ( email )

Ellinor Marving

Statens Serum Institut ( email )

Arieh Cohen

affiliation not provided to SSRN ( email )

No Address Available

Nicolai Balle Larsen

affiliation not provided to SSRN ( email )

No Address Available

Troels Lillebaek

affiliation not provided to SSRN ( email )

No Address Available

Henrik Ullum

Statens Serum Institut ( email )

Kåre Mølbak

affiliation not provided to SSRN ( email )

No Address Available

Tyra Grove Krause

Statens Serum Institut

Denmark

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