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Observational Study
. 2023 Oct 11;28(1):421.
doi: 10.1186/s40001-023-01373-3.

Clinical and experimental evidence suggest omicron variant of SARS-CoV-2 is inherently less pathogenic than delta variant independent of previous immunity

Ramachandran Thiruvengadam #  1 , Zaigham Abbas Rizvi #  1 , Sreevatsan Raghavan  1 , Deepika Rathna Murugesan  1 , Mudita Gosain  1 , Jyotsna Dandotiya  1 , Ayushi  1 , Sweety Samal  1 , Anil K Pandey  2 , Nitya Wadhwa  1 , Shinjini Bhatnagar  1 , Amit Awasthi  3 , Pramod Kumar Garg  4
Affiliations
Observational Study

Clinical and experimental evidence suggest omicron variant of SARS-CoV-2 is inherently less pathogenic than delta variant independent of previous immunity

Ramachandran Thiruvengadam et al. Eur J Med Res. .

Abstract

Objectives: To study clinical disease outcomes in both human and animal models to understand the pathogenicity of omicron compared to the delta variant.

Methods: In this cross-sectional observational study, clinical outcomes of adults who tested positive at 2 testing centres in Delhi National Capital Region between January 2022 and March 2022 (omicron-infected; N = 2998) were compared to a similar geographical cohort (delta-infected; N = 3292). In addition, disease course and outcomes were studied in SARS-CoV-2-infected golden Syrian hamsters and K-18 humanized ACE2 transgenic mice.

Results: Omicron variant infection was associated with a milder clinical course [83% (95% CI 61, 94) reduced risk of severity compared against delta] adjusting for vaccination, age, sex, prior infection and occupational risk. This correlated with lower disease index and vir comparing omicron with other variants in animal models.

Conclusions: Infections caused by the omicron variant were milder compared to those caused by the delta variant independent of previous immunity.

Keywords: COVID-19; Omicron; SARS-CoV-2; hACE2 transgenic mice; hamster.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1
Study flow for the selection of the omicron cohort
Fig. 2
Fig. 2
Pathological manifestations of Omicron (B.1.1.529) infection in Syrian hamster and hACE2.Tg mice. Pathological manifestations of intranasal Omicron (B.1.1.529) infection was evaluated and compared with ancestral Wuhan (nCoV-2019) and Delta (B.1.617.2) strain infection or uninfected (UI) in Syrian hamster and hACE2.Tg mice. The changes in body mass was plotted as percentage of the day 0 body mass till day 14 or 6 post infection, respectively, for (A) hamster and (F) hACE2.Tg mice. The lungs of the sacrificed animals were harvested and images and thereafter, viral load and histopathology of the lungs were studied. B and G Shows representative lungs from individual groups at 4 dpi and 6 dpi from hamster and hACE2.Tg mice, respectively. C and H Viral load of the lungs at 4dpi and 6 dpi from hamster and hACE2.Tg mice, respectively. D–E and I–J Histopathological assessment of the HE stained lungs were carried out by trained pathologist by blinded scoring on the scale of 0–5 (where 0 described no feature and 5 described highest pathological feature) of the lungs at 4 dpi and 6 dpi from hamster and hACE2.Tg mice, respectively. Each experiment was carried out with n = 5 animals and replicated 3 times independently. One-way ANOVA using non-parametric Kruskal–Wallis test for multiple comparison. ns = non-significant, *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001
See this image and copyright information in PMC

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