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

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  • Peter Bager, Statens Serum Institut
  • ,
  • Jan Wohlfahrt, Department of Epidemiology Research, Statens Serum Institut
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  • Samir Bhatt, Københavns Universitet, Imperial College London
  • ,
  • Marc Stegger, Statens Serum Institut
  • ,
  • Rebecca Legarth, Statens Serum Institut
  • ,
  • Camilla Holten Møller, Infectious Disease Preparedness, Statens Serum Institut
  • ,
  • Robert Leo Skov, Statens Serum Institut
  • ,
  • Palle Valentiner-Branth, Statens Serum Institut
  • ,
  • Marianne Voldstedlund, Infectious Disease Preparedness, Statens Serum Institut
  • ,
  • Thea K. Fischer, Københavns Universitet
  • ,
  • Lone Simonsen, Department of Natural Science and Environment
  • ,
  • Nikolai Søren Kirkby, Københavns Universitet
  • ,
  • Marianne Kragh Thomsen
  • Katja Spiess, Statens Serum Institut
  • ,
  • Ellinor Marving, Statens Serum Institut
  • ,
  • Nicolai Balle Larsen, Statens Serum Institut
  • ,
  • Troels Lillebaek, Statens Serum Institut, Københavns Universitet
  • ,
  • Henrik Ullum, Statens Serum Institut
  • ,
  • Kåre Mølbak, Statens Serum Institut, Københavns Universitet
  • ,
  • Tyra Grove Krause, Statens Serum Institut
  • ,
  • the Omicron-Delta study group

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.

OriginalsprogEngelsk
TidsskriftThe Lancet Infectious Diseases
Vol/bind22
Nummer7
Sider (fra-til)967-976
Antal sider10
ISSN1473-3099
DOI
StatusUdgivet - jul. 2022

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