New Studies Suggest Omicron Infections Are Less Severe Than Delta Ones

Brenda Goodman, MA

December 22, 2021

Editor's note: Find the latest COVID-19 news and guidance in Medscape's Coronavirus Resource Center.

People who get COVID-19 infections caused by the Omicron variant are less likely to need hospital care, compared with those infected by the Delta variant, according to two large new studies from the U.K. and South Africa.

The findings, which were released ahead of peer review, add to previous glimmers of evidence suggesting that Omicron — while extremely contagious — may result in less severe symptoms than its predecessors.

"This is helping us quantify how much less severe Omicron is than Delta, and it appears to be between 40 to 75% reduced risk of hospitalizations, adjusted for many factors, which is very good," said Eric Topol, MD, the editor-in-chief of Medscape and a cardiologist at Scripps Research Translational Institute in La Jolla, CA.

The first analysis, which was done by the World Health Organization Collaborating Centre for Infectious Disease Modelling and Imperial College London, found that overall, people infected by Omicron had about a 20% reduced risk of needing any hospital care for their infections and a 40% lower risk of an overnight hospital stay, compared to those infected with Delta.

Meanwhile, people who were re-infected — meaning they caught Omicron after recovering from a previous COVID-19 infection — had a 50%-60% lower risk of needing hospital care, likely reflecting the benefits of having some prior immunity against the same family of viruses.

The study included everyone with polymerase chain reaction-confirmed COVID-19 in the U.K. during the first 2 weeks of December — roughly 56,000 Omicron cases and 269,000 Delta infections.

The second study, from researchers at the National Institute for Communicable Diseases in South Africa, included more than 29,000 COVID-19 cases that had lab results highly suggestive of Omicron infections. Compared to people infected with the Delta variant, those with presumed Omicron infections were about 70% less likely to have severe disease.

While the news is hopeful for people on a population level, health care systems may still be stressed, the study authors warned.

 "Given the high transmissibility of the Omicron virus, there remains the potential for health services to face increasing demand if Omicron cases continue to grow at the rate that has been seen in recent weeks," said study author Neil Ferguson, PhD, who studies how infectious diseases spread at Imperial College London.

The study authors say their findings are specific to the U.K. and South Africa, where substantial portions of the population have some immune protection from past infection. In other words, they may not apply to countries where fewer people have been vaccinated or recovered from a bout with COVID-19.


Eric Topol, MD, editor-in-chief, Medscape; cardiologist, Scripps Research Translational Institute, La Jolla, CA.

Neil Ferguson, PhD, professor, Imperial College London.

MRC Center for Global Infectious Disease Analysis, Imperial College London: "Report 50 — Hospitalisation risk for Omicron cases in England."

MedRxiv: "Early assessment of the clinical severity of the SARS-CoV-2 Omicron variant in South Africa."


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