More Evidence Omicron May Be Weaker Than Prior Variants

A growing body of data further strengthened the case for Omicron being a less severe variant than Delta, said Anthony Fauci, MD, chief medical advisor to President Biden, during a White House COVID-19 Response Team phone briefing on Wednesday.

Given certain caveats, “all indications point to a lesser severity of Omicron versus Delta,” Fauci said.

However, he also stressed that the latest data are still “preliminary,” and gleaned from patients in South Africa and the U.K., which may not directly translate to the “demographically diverse” U.S. population.

Fauci highlighted a study published in the International Journal of Infectious Diseases, which compared COVID-19 hospital admissions at a large hospital in the city of Tshwane in the Gauteng Province of South Africa from Nov. 14, 2021 and on (Omicron period) versus prior admissions starting in May 4, 2020 (pre-Omicron period).

Of the 466 cases during the recent Omicron-led wave, 4.5% of patients died compared with 21.3% of the 3,976 cases in previous waves. ICU admissions occurred among 1% versus 4.3%, respectively, and length of hospital stay was 4.0 days versus 8.8 days. Mean age of patients dropped to 39 years from 49 years.

Only 45% of patients in the recent wave required supplemental oxygen compared with 99.5% in the first wave, Fauci noted.

This “decoupling” in the rate of cases, hospitalizations, and deaths in comparison to previous waves “corroborat[es] the clinical findings of milder omicron disease in the hospital,” the authors wrote.

The “changing clinical presentation of SARS-CoV-2” is likely attributable to the high levels of prior infection in the country and, “to some extent,” to vaccines, Fauci suggested, although vaccination levels in South Africa are “relatively low.”

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In addition, recent data from the U.K. Health Security Agency found about a 60% reduced risk of hospital admission with Omicron compared with Delta (HR 0.38, 95% CI 0.30-0.50) based on 70 hospital admissions with Omicron and 3,884 with Delta.

While “preliminary,” population data from Scotland from November 1 to December 19 also showed a “two-thirds reduction in the risk of COVID-19 hospitalization” with the Omicron variant, Fauci noted.

Additionally, in vitro and animal studies that sought to determine whether there has been an “inherent lowering of virulence of the virus” showed that while Omicron replicates faster in the bronchus, researchers in Hong Kong found less efficient replication in the lungs. Modeling in hamsters at the University of Tokyo also found that “Omicron poorly infects and spreads in the lung and is less pathogenic compared to Delta,” Fauci said. Similar studies at the NIH’s Vaccine Research Center in non-human primates are currently underway.

Shifting gears, CDC Director Rochelle Walensky, MD, MPH, provided an overview of Omicron’s impact in the U.S. The 7-day average daily case rate jumped to 240,000 this week, representing a 60% increase over the previous week.

However, the 7-day average for hospitalizations was about 9,000, and the 7-day average for deaths was roughly 1,100, reflecting a 14% increase and a 7% decrease, respectively, over the week prior, she said.

One reason that hospitalizations may look “comparatively low” in contrast to rising case rates is that hospitalizations typically lag by about 2 weeks, or it could in fact be that the Omicron variant causes milder disease, “especially among the vaccinated and the boosted,” as the preliminary studies from the U.K. and South Africa suggested, she added.

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Fauci noted that because more children are being infected with the “highly transmissible” Omicron variant, more have been hospitalized.

However, he pointed to another wrinkle that might further bolster the case for Omicron being milder: “Many children are hospitalized with COVID as opposed to because of COVID, reflecting the high degree of penetrance of infection among the pediatric population. The final conclusion about the level of severity in children remains to be determined.”

Last Updated December 29, 2021

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    Shannon Firth has been reporting on health policy as MedPage Today’s Washington correspondent since 2014. She is also a member of the site’s Enterprise & Investigative Reporting team. Follow

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