Studies suggest less severe outcomes with Omicron


Dr Rahul Pandit, Director, Critical Care, Fortis hospital, Mumbai; Member, COVID-19 Task Force, Maharashtra Government    24 December 2021

A preprint study from Scotland has shown that infection with Omicron is associated with fewer hospitalizations with Omicron when compared to delta infections. It has further shown that the booster dose provides considerable protection within two weeks of administration compared to two doses received 25 or more weeks ago.1

In this preliminary study, researchers from the University of Edinburgh, University of Strathclyde and Public Health Scotland analysed data from 23,840 patients with Omicron infection and 126,511 patients infected with the Delta variant. Data was collected between 1st November, 2021 to 19th December, 2021. S gene was used to differentiate between Delta and Omicron cases; a negative S gene was a marker of Omicron infection, while S gene positive status indicated Delta infection. Vaccination status was determined on the day of the positive RT-PCR test. Scotland recorded its first Omicron case on 23rd November. By the 19th of December there were 23,840 S gene negative cases and 126,511 S gene positive cases. Almost half of the S gene negative cases (11,732; 49.2%) were reported in the age group 20-39 years.

Compared to the number of delta cases, likely reinfection cases were 10 times higher with the Omicron variant. The possible reinfections among S gene negative cases were 7.6% versus 0.7% in S gene positive cases. Most of the hospitalizations were among S gene positive cases. There were 856 hospitalizations among delta cases in comparison to 15 Omicron cases. The third dose of the vaccine (booster dose with Pfizer or Moderna vaccine) was associated with a 57% reduction in the risk of developing symptomatic Omicron infection relative to ≥25 weeks after the second dose of the vaccine (Pfizer-BioNTech or Oxford-AstraZeneca or Moderna).

An early assessment of the clinical severity of the Omicron variant in South Africa from the National Institute for Communicable Diseases was also posted on the preprint site medRxiv on Tuesday. This analysis too suggested 80% reduced risk of hospitalization as well as 70% lower risk of severe disease among Omicron infections compared to delta cases. This reduction in disease severity has been attributed to high levels of immunity among the population owing to natural infection and/or vaccination.2

Another study from the Imperial College London and the WHO Collaborating Centre for Infectious Disease Modelling showed overall 20% lower odds of omicron-related hospitalization and 40% lower risk of hospital stay for a day or longer compared to patients with delta infection. Among reinfection cases, the risk of hospital admission was reduced by 50-60%.3

These studies show that disease outcomes are less severe with Omicron infection when compared to delta infections. That Omicron causes a “mild” disease seems reassuring. However, let’s not lose sight of the fact that it is a highly transmissible variant and is 4.2 times more contagious in the early stages than the delta variant. There is a 5.4-fold increased risk of reinfection with the Omicron variant compared to the currently dominant Delta variant, as shown in a study from the Imperial College London. The current study too has shown a 10 times higher reinfection rate with Omicron vis a vis Delta variant. The doubling time is 1.5 to 3 days in countries with community transmission, as per the WHO. Therefore, a surge of cases, should the third wave materialize, may well overwhelm hospitals and resources.

“Prevention is always better than cure”…Our best bet against a looming third wave is to strictly comply with Covid-appropriate behavior. Double mask with well-fitting masks, avoid poorly ventilated spaces and crowded public places, hand hygiene and get vaccinated, if still not taken. These studies also support a booster dose of Covid-19 vaccines. Its time for a booster dose.


  1. Sheikh A, Kerr S, Woolhouse M, McMenamin J, Robertson C. Severity of Omicron variant of concern and vaccine effectiveness against symptomatic disease: national cohort with nested test negative design study in Scotland. Published - 22 Dec 2021. https://www.research.ed.ac.uk/en/publications/severity-of-omicron-variant-of-concern-and-vaccine-effectiveness-.
  2. Wolter N, et al. Early assessment of the clinical severity of the SARS-CoV-2 Omicron variant in South Africa. medRxiv, Posted December 21, 2021. doi: https://doi.org/10.1101/2021.12.21.21268116.
  3. Ferguson N, Ghani A, Hinsley W, Volz E, et al. Report 50 - Hospitalisation risk for Omicron cases in England. 22 December 2021. https://www.imperial.ac.uk/mrc-global-infectious-disease-analysis/covid-19/report-50-severity-omicron/.

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