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Omicron is not really milder: 20% become severe after hospitalization
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20% patients related to Omicron become severe after hospitalization.
Omicron is milder? “The Lancet” study of tens thousands of people: 20% become severe after hospitalization.
The Omicron variant is spreading rapidly around the world, and understanding the severity of symptoms caused by Omicron infection can help in better public health planning and response.
Recently, “The Lancet” published a large-scale study from South Africa, evaluating the clinical manifestations of Omicron infection by analyzing more than 30,000 recent nucleic acid-positive cases.
Screenshot source: The Lancet
The results of the study showed that compared with other infections during the same period, the risk of hospitalization for Omicron-infected patients was reduced by 80%; compared with earlier Delta (Delta) infections, Omicron-infected patients had a 70% lower risk of severe disease.
In addition, it is worth noting that in the generally young South African population, 21% of hospitalized patients infected with the Omicron variant also developed severe symptoms.
The same period review article in The Lancet also pointed out that “Omicron may be milder (than other variants), but not absolutely mild (milder but not mild)”.
In the article, two experts from the Li Ka Shing Faculty of Medicine of the University of Hong Kong and the WHO Collaborating Centre for Infectious Disease Epidemiology and Control commented, “Even if the average clinical presentation is mild, it may change with time. There is a considerable societal burden as infection rates increase, including disease burden, lost productivity, suffering, and additional pressure on the healthcare system.”
The study was led by a team from South Africa’s National Institute of Infectious Diseases. A total of 161,300 COVID-19 cases were reported in South Africa between October 1 (week 39) – December 6 (week 49), 2021.
Among them, 38,282 people were diagnosed by nucleic acid detection, and 29,721 cases of Omicron infection and 1,412 cases of non-Omicron infection were identified according to the sequencing data.
A surrogate indicator of Omicron infection is SARS-CoV-2 Spike (S) protein gene target detection failure (SGTF, known to be caused by a mutation in the OmicronS gene).
A contemporaneous review article discussed that it is reasonable to use this as a surrogate indicator, considering that other prevalent variants did not have this feature during the study period.
The proportion of Omicron infections increased from 3.2% (2 cases/63 cases) in the week of October 1 to 97.9% (21978 cases/22455 cases) in the week of November 30th.
The number of COVID-19 cases in South Africa and the proportion of Omicron infections (Image source: Reference )
Omicron infection vs non-Omicron infection at the same time
The research team first compared disease severity in Omicron-infected and non-Omicron-infected individuals between October 1 and November 30, 2021.
After adjusting for other factors that may affect the risk of hospitalization, the probability of hospitalization in patients with Omicron infection (2.4% [256 cases/10547 cases]) was significantly lower than that in patients with other infections (12.8% [121 cases/948 cases]), and the risk of hospitalization was reduced by 80% (aOR: 0.2, 95%CI 0.1–0.3).
Once hospitalized, the probability of severe illness was similar between the different variants (Omicron 21% [42/204 cases vs 40% [45/113 cases] for other infections], aOR: 0.7, 95%CI 0.3–1.4 ). The paper notes that, given the small number of people included in this analysis, it is currently impossible to draw firm conclusions about the risk of severe illness in patients hospitalized with Omicron infection.
▲The number of Delta-infected and Omicron-infected patients (surrogate indicator is SGTF) in the 13th week (April 1st)-49th week (December 6th) of 2021 in hospitalized patients with COVID-19 with clear sequencing results in South Africa ( Image source: Reference )
Omicron infection vs early Delta infection
To further assess the severity of the disease caused by Omicron, the research team continued to compare data from patients infected with the Delta variant diagnosed earlier (April 1-November 9, 2021).
After adjusting for other severe disease-related factors, Omicron-infected patients had a significantly lower 70% chance of severe disease (23.4%) than early Delta-infected patients (62.5%) (aOR: 0.3, 95% CI 0.2–0.5).
Whether Omicron itself is “toxic” is still a question mark
The paper also points out that the observed reduction in the severity of symptoms in Omicron-infected patients may be due to pre-existing immunity in infected individuals, including immunity from previous infections and vaccinations.
Therefore, the likelihood and extent of the reduced “virulence” of the Omicron variant itself is unclear.
Assessing the severity of disease caused by Omicron variants in countries with different levels of previous infection and vaccination levels will help us to more fully understand the impact of Omicron.
A contemporaneous review article emphasized that in the context of an expanding prevalence of Omicron variants, “generalized data from South Africa” ”has crucial global implications” for reference in other regions, although this study found that Omicron caused less symptoms than Delta, But “we should not assume that the health effects of Omicron epidemics are lower elsewhere”.
 Nicole Wolter, et al., (2022). Early assessment of the clinical severity of the SARS-CoV-2 omicron variant in South Africa: a data linkage study. The Lancet, DOI: https://doi.org /10.1016/S0140-6736(22)00017-4
 Joshua Nealon, Benjamin J Cowling. (2022). Omicron severity: milder but not mild. Lancet, DOI: https://doi.org/10.1016/S0140-6736(22)00056-3
 WHO Technical Advisory Group on COVID-19 Vaccine Components (TAG-CO-VAC) Interim Statement on COVID-19 Vaccines in the Context of Variant Transmission of SARS-CoV-2 in Omicron. Retrieved January 28, 2022 from https://www.who.int/en/news/item/11-01-2022-interim-statement-on-covid-19-vaccines-in-the-context-of-the-circulation-of-the -omicron-sars-cov-2-variant-from-the-who-technical-advisory-group-on-covid-19-vaccine-composition
 Enhancing response to Omicron SARS-CoV-2 variant. Retrieved January 28, 2022 from https://www.who.int/publications/m/item/enhancing-readiness-for-omicron-(b.1.1.529 )-technical-brief-and-priority-actions-for-member-states
20% patients related to Omicron become severe after hospitalization
(source:internet, reference only)