Risk of hospitalisation associated with infection with SARS-CoV-2 omicron variant versus delta variant in Denmark: an observational cohort study - 23/06/22

Doi : 10.1016/S1473-3099(22)00154-2

Peter Bager, PhD ^a,⁎ , Jan Wohlfahrt, DMSc ^b, Samir Bhatt, DPhil ^g,^k,^l, Marc Stegger, PhD ^c, Rebecca Legarth, PhD ^a, Camilla Holten Møller, PhD ^a, Robert Leo Skov, MD ^a, Palle Valentiner-Branth, PhD ^a, Marianne Voldstedlund, PhD ^a, Thea K Fischer, ProfMD ^h,^m, Lone Simonsen, PhD ⁿ, Nikolai Søren Kirkby, PhD ^o, Marianne Kragh Thomsen, MD ^p, Katja Spiess, PhD ^d, Ellinor Marving, PhD ^d, Nicolai Balle Larsen, PhD ^d,^e, Troels Lillebaek, ProfDMSc ^f,ⁱ, Henrik Ullum, ProfPhD ^a, Kåre Mølbak, DMSc ^a,^j, Tyra Grove Krause, PhD ^a

the Omicron-Delta study group^{^†}
Authors in the Omicron-Delta study group are listed in the Supplementary Material

Sofie Marie Edslev, Raphael Niklaus Sieber, Anna Cäcilia Ingham, Maria Overvad, Mie Agermose Gram, Frederikke Kristensen Lomholt, Louise Hallundbæk, Caroline Hjorth Espensen, Sophie Gubbels, Marianne Karakis, Karina Lauenborg Møller, Stefan Schytte Olsen, Zitta Barrella Harboe, Caroline Klint Johannesen, Maarten van Wijhe, Jon Gitz Holler, Ram Benny Christian Dessau, Martin Barfred Friis, David Fuglsang-Damgaard, Mette Pinholt, Thomas Vognbjerg Sydenham, John Eugenio Coia, Ea Sofie Marmolin, Anders Fomsgaard, Jannik Fonager, Morten Rasmussen, Arieh Cohen

^a Division of Infectious Disease Preparedness, Statens Serum Institut, Copenhagen, Denmark

^b Department of Epidemiology Research, Statens Serum Institut, Copenhagen, Denmark

^c Department of Bacteria, Parasites, and Fungi, Statens Serum Institut, Copenhagen, Denmark

^d Department of Virus & Microbiological Special Diagnostics, Statens Serum Institut, Copenhagen, Denmark

^e Test Center Danmark, Statens Serum Institut, Copenhagen, Denmark

^f International Reference Laboratory of Mycobacteriology, Statens Serum Institut, Copenhagen, Denmark

^g Section of Epidemiology, University of Copenhagen, Copenhagen, Denmark

^h Department of Public Health, University of Copenhagen, Copenhagen, Denmark

ⁱ Global Health Section, University of Copenhagen, Copenhagen, Denmark

^j Department of Veterinary and Animal Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark

^k MRC Centre for Global Infectious Disease Analysis, School of Public Health, Imperial College London, London, UK

^l The Abdul Latif Jameel Institute for Disease and Emergency Analytics, School of Public Health, Imperial College London, London, UK

^m Department of Clinical Research, Nordsjaellands Hospital, Hillerød, Denmark

ⁿ PandemiX Center, Department of Science and Environment, Roskilde University, Roskilde, Denmark

^o Department of Clinical Microbiology, Copenhagen University Hospital, Rigshospitalet, Copenhagen, Denmark

^p Department of Clinical Microbiology, Aarhus University Hospital, Aarhus, Denmark

^* Correspondence to: Dr Peter Bager, Division of Infectious Disease Preparedness, Statens Serum Institut, 2300 Copenhagen, Denmark Division of Infectious Disease Preparedness Statens Serum Institut Copenhagen 2300 Denmark

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Summary

Background

Estimates of the severity of the SARS-CoV-2 omicron variant (B.1.1.529) are crucial to assess the public health impact associated with its rapid global dissemination. We estimated the risk of SARS-CoV-2-related hospitalisations after infection with omicron compared with the delta variant (B.1.617.2) in Denmark, a country with high mRNA vaccination coverage and extensive free-of-charge PCR testing capacity.

Methods

In this observational cohort study, we included all RT-PCR-confirmed cases of SARS-CoV-2 infection in Denmark, with samples taken between Nov 21 (date of first omicron-positive sample) and Dec 19, 2021. Individuals were identified in the national COVID-19 surveillance system database, which included results of a variant-specific RT-PCR that detected omicron cases, and data on SARS-CoV-2-related hospitalisations (primary outcome of the study). We calculated the risk ratio (RR) of hospitalisation after infection with omicron compared with delta, overall and stratified by vaccination status, in a Poisson regression model with robust SEs, adjusted a priori for reinfection status, sex, age, region, comorbidities, and time period.

Findings

Between Nov 21 and Dec 19, 2021, among the 188 980 individuals with SARS-CoV-2 infection, 38 669 (20·5%) had the omicron variant. SARS-CoV-2-related hospitalisations and omicron cases increased during the study period. Overall, 124 313 (65·8%) of 188 980 individuals were vaccinated, and vaccination was associated with a lower risk of hospitalisation (adjusted RR 0·24, 95% CI 0·22–0·26) compared with cases with no doses or only one dose of vaccine. Compared with delta infection, omicron infection was associated with an adjusted RR of hospitalisation of 0·64 (95% CI 0·56–0·75; 222 [0·6%] of 38 669 omicron cases admitted to hospital vs 2213 [1·5%] of 150 311 delta cases). For a similar comparison by vaccination status, the RR of hospitalisation was 0·57 (0·44–0·75) among cases with no or only one dose of vaccine, 0·71 (0·60–0·86) among those who received two doses, and 0·50 (0·32–0·76) among those who received three doses.

Interpretation

We found a significantly lower risk of hospitalisation with omicron infection compared with delta infection among both vaccinated and unvaccinated individuals, suggesting an inherent reduced severity of omicron. Our results could guide modelling of the effect of the ongoing global omicron wave and thus health-care system preparedness.

Funding

None.

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Plan

Introduction

Methods

Study design and population

Data sources

COVID-19 hospital admission (primary outcome)

Covariates

Statistical analysis

Role of the funding source

Results

Discussion

Data sharing

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Vol 22 - N° 7

P. 967-976 - juillet 2022 Retour au numéro

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Risk of hospitalisation associated with infection with SARS-CoV-2 omicron variant versus delta variant in Denmark: an observational cohort study - 23/06/22

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