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Why Omicron COVID-19 Cases Are Usually Less Severe Than Delta Cases

The fast-spreading Omicron variant of COVID-19 seems to cause less severe illness than previous versions of the virus.

The reason why may be how the latest form of the novel coronavirus behaves in the body.

A new study has found that people in the United States who develop COVID-19 for the first time from the now dominant Omicron variant are less likely to become seriously ill than those who became sick with the Delta variant.

The research, which was overseen by scientists from Case Western Reserve University School of Medicine in Ohio, reported a 56 percent reduction in risk of hospitalization, a 66 percent reduction in intensive care unit admissions, and an 84 percent reduction in need for mechanical ventilation among people sick with the Omicron variant, compared to people who were ill with the Delta variant.

“There’s definitely been a reduction in the percentage of people who need to be hospitalized or go to the [emergency room],” Dr. Pamela B. Davis, a study co-author and a professor of medicine at Case Western, told Healthline.

Reductions in severity of illness with the Omicron variant were seen across age groups, noted Davis. This included young children who are unvaccinated as well as older, vaccinated individuals generally seen as at risk for more severe COVID-19 symptoms.

Dr. Shruti K. Gohil, an associate medical director of infection prevention and epidemiology at the University of California Irvine, said that there may be a simple explanation for the less serious cases.

“The Omicron variant is seven times more efficient in replicating in the upper airways” than previous variants of the coronavirus, Gohil told Healthline.

However, the Omicron variant tends to remain in the upper respiratory system rather than replicating in the lungs.

“That’s what makes it tremendously contagious but also less severe,” Gohil said.

The study by the numbers

Researchers analyzed data from more than half a million people who developed COVID-19 between September and December 2021, including 14,000 confirmed to have contracted the Omicron variant in late December.

A follow-up analysis conducted in January, when more than 90 percent of new COVID-19 cases in the United States were attributed to the Omicron variant, reaffirmed the conclusions, according to Rong Xu, PhD, a researcher at Case Western.

“The general trend of the Omicron variant is toward less severe risk of hospitalization,” Xu told Healthline.

She noted that even among people with comorbid illnesses such as type 2 diabetes and organ transplants, the Omicron variant is “still milder than a Delta variant infection.”

The U.S. findings are consistent with those from international researchers who have studied Omicron variant outbreaks in other countries such as Great Britain and South Africa.

What higher hospitalization numbers mean

COVID-19 hospitalizations have reached an all-time high in the United States with more than 120,000 people in hospitals as of Jan. 11.

However, that’s a reflection of the vast number of people who’ve contracted the Omicron variant, Gohil said.

She noted that anecdotal evidence gathered in patient care settings generally aligns with the research findings: Individuals who do end up in the hospital due to the Omicron variant are likely to either be unvaccinated or those with underlying health conditions exacerbated by COVID-19.

“Most severely ill people are unvaccinated,” Gohil said.

She noted that hospitalized people with pre-existing conditions such as diabetes or heart disease include vaccinated individuals with COVID-19 infections.

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