Review confirms reduced severity of SARS-CoV-2 Omicron infection

In a recent study posted to the Research Square preprint* server, researchers review existing literature to quantify the transmissibility, immune evasion, reinfection, and severity of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant.

Study: Omicron SARS-CoV-2 Variant of Concern: A Review on its Transmissibility, Immune Evasion, Reinfection, and Severity. Image Credit: M.Aka /

As a result of its high transmissibility, the SARS-CoV-2 Omicron variant is rapidly spreading across the world. In the United States, for example, this strain of SARS-CoV-2 is responsible for 95% of all new coronavirus disease 2019 (COVID-19) cases since January 1, 2022.

Extensive research is needed to understand the extent to which the increased transmissibility and virulence of Omicron threaten public health globally. Furthermore, it is also crucial to determine how the global population should recognize these dynamics, perceive risk, and adhere to public health and social measures amid the emergence of the Omicron variant.

About the study

In the present study, the researchers performed a literature search in PubMed, Web of Science, Scopus, ScienceDirect, Google Scholar, medRxiv, and bioRxiv using keywords such as Omicron, transmissibility, immune evasion, reinfection, and severity to find articles published in 2021 and 2022. Data reviewed in the present study included all the existing articles having at least one of these keywords.

Study findings

In one Norwegian study, among 117 people in a party that was 96% fully vaccinated, 74% got infected with Omicron after coming in contact with one individual from South Africa, from where Omicron was originally detected. Moreover, a study conducted in the United Kingdom found that the Omicron variant had a 10 times greater risk of reinfection than the Delta variant.

In another study conducted in South Africa, substantial population-level evidence was acquired which demonstrated that Omicron, in contrast to the Beta and Delta variants, evades immunity from prior infection. Similarly, in a large-scale study conducted in Denmark, the researchers found that although there was no significant difference in transmissibility for Omicron and Delta among unvaccinated people, the Omicron variant was 2.6-3.7 times more infectious than the Delta variant among vaccinated people.

Among studies discussing the severity of Omicron infections, one study conducted by researchers from the University of Edinburgh in Scotland indicated that when compared to the Delta variant, Omicron resulted in a two-thirds reduction in the probability of COVID-19 hospitalization. Notably, existing literature demonstrated that Omicron infections are much less severe than Delta variant infections.

When assessing the clinical severity of the SARS-CoV-2 Omicron variant using nationwide data, researchers in South Africa found that people with Omicron infection had an 80% lower risk of hospitalization than those without Omicron infection.

In another study, the researchers compared the clinical characteristics of 466 patients infected with Omicron and admitted to a hospital in Tshwane, South Africa to 3,962 hospital admissions from earlier COVID-19 waves. To this end, the length of hospital stay was only 4.0 days for Omicron infection, which was comparable to the average hospital stay of 8.8 days during earlier waves.


Taken together, the current study revealed that Omicron spreads faster than previous SARS-CoV-2 variants. This is likely due to the ability variant of this to evade immune responses induced by vaccination and previous infections.

Omicron also has several times higher odds of infecting vaccinated and previously infected people than Delta and other SARS-CoV-2 variants. However, the risk of infection from Delta and Omicron is almost similar among unvaccinated people.

Almost all the studies reviewed in the current work consistently showed that the severity of infection in Omicron cases is much lower compared to Delta and other previous variants. Subsequently, there is a considerably lower risk of requiring hospital admission, intensive care unit (ICU) admission, and mechanical ventilators in Omicron-infected individuals. Furthermore, Omicron infections were associated with shorter hospital stays and lower mortality rates. Although Omicron has an enhanced ability to escape immunity developed by vaccines, booster doses significantly protect individuals from symptomatic COVID-19 infections.

Therefore, the authors strongly recommend the continuance of usual prevention measures, such as vaccination, masking, and suitable infection mitigation strategies for decreasing Omicron transmission, reducing morbidity and death to ultimately reduce the workload of healthcare systems globally.

*Important notice

Research Square publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
  • Mohsin, M., & Mahmud, S. (2022). Omicron SARS-CoV-2 Variant of Concern: A Review on its Transmissibility, Immune Evasion, Reinfection, and Severity. Research Square. doi:10.21203/  

Posted in: Medical Research News | Disease/Infection News

Tags: Coronavirus, Coronavirus Disease COVID-19, covid-19, Healthcare, Hospital, immunity, Intensive Care, Mortality, Omicron, Public Health, Research, Respiratory, SARS, SARS-CoV-2, Severe Acute Respiratory, Severe Acute Respiratory Syndrome, Syndrome, Vaccine

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Neha Mathur

Neha Mathur has a Master’s degree in Biotechnology and extensive experience in digital marketing. She is passionate about reading and music. When she is not working, Neha likes to cook and travel.

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