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. 2023 Apr:129:135-141.
doi: 10.1016/j.ijid.2023.01.027. Epub 2023 Jan 26.

Reduction of the risk of severe COVID-19 due to Omicron compared to Delta variant in Italy (November 2021 - February 2022)

Daniele Petrone  1 , Alberto Mateo-Urdiales  2 , Chiara Sacco  3 , Flavia Riccardo  3 , Antonino Bella  3 , Luigina Ambrosio  3 , Alessandra Lo Presti  3 , Angela Di Martino  3 , Emiliano Ceccarelli  3 , Martina Del Manso  3 , Massimo Fabiani  3 , Paola Stefanelli  3 , Patrizio Pezzotti  3 , AnnaTeresa Palamara  3 ; Italian Integrated Surveillance of COVID-19
Affiliations

Reduction of the risk of severe COVID-19 due to Omicron compared to Delta variant in Italy (November 2021 - February 2022)

Daniele Petrone et al. Int J Infect Dis. 2023 Apr.

Abstract

Objectives: During 2022, Omicron became the dominant SARS-CoV-2 variant in Europe. This study aims to assess the impact of such variant on severe disease from SARS-CoV-2 compared with the Delta variant in Italy.

Methods: Using surveillance data, we assessed the risk of developing severe COVID-19 with Omicron infection compared with Delta in individuals aged ≥12 years using a multilevel negative binomial model adjusting for sex, age, vaccination status, occupation, previous infection, weekly incidence, and geographical area. We also analyzed the interaction between the sequenced variant, age, and vaccination status.

Results: We included 21,645 cases of SARS-CoV-2 infection where genome sequencing found Delta (10,728) or Omicron (10,917), diagnosed from November 15, 2021 to February 01, 2022. Overall, 3,021 cases developed severe COVID-19. We found that Omicron cases had a reduced risk of severe COVID-19 compared with Delta cases (incidence rate ratio [IRR] = 0.77; 95% confidence interval [CI]: 0.70-0.86). The largest difference was observed in cases aged 40-59 (IRR = 0.66; 95% CI: 0.55-0.79), while no protective effect was found in those aged 12-39 (IRR = 1.03; 95% CI: 0.79-1.33). Vaccination was associated with a lower risk of developing severe COVID-19 in both variants.

Conclusion: The Omicron variant is associated with a lower risk of severe COVID-19 compared to infection with the Delta variant, but the degree of protection varies with age.

Keywords: COVID-19; Death; Delta variant; Hospitalization; Omicron variant; SARS-CoV-2.

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Conflict of interest statement

The authors have no competing interests to declare.

Figures

Figure 1
Figure 1
Adjusted IRR estimated with the negative binomial linear mixed model (model 1) *The IRR for normalized incidence, obtained as the difference between observed incidence and mean incidence and divided by the standard deviation of the overall incidence, is expressed as a unit-increase of the standard deviation. CI, confidence interval; HCW, healthcare workers; IRR, incidence rate ratio.
See this image and copyright information in PMC

References

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Supplementary concepts