The Omicron variant of the coronavirus is much less likely to cause hospitalization and death than the earlier Delta variant, a study released by the Centers for Disease Control and Prevention finds, confirming to a dramatic degree predictions some virologists have been making since the now dominant variant first appeared in Southern Africa.

The findings provided “key insight” into how the newest variant operates, CDC Director Dr. Rochelle Walensky said as she introduced the data, which came from a study of both Omicron and Delta infections throughout Southern California. The study found that, as compared to the Delta variant, Omicron resulted in a 91 percent drop in the risk of death, while the risk of hospitalization was halved. For those who did require hospitalization from Omicron, risk of being admitted to an intensive care unit — signaling more serious disease — was reduced by three-fourths.

Omicron infection is “associated with substantially reduced risk of severe clinical endpoints and shorter durations of hospital stay” compared to Delta, the authors found. Of the nearly 70,000 people in the study group (50,000 of whom tested positive for Omicron), not a single patient admitted to the hospital while battling the Omicron variant required ventilation. And the average hospital stay for an Omicron patient was only three days.

“It is going to be less severe,” Dr. Anthony Fauci, the Biden administration’s top medical adviser, said of the Omicron variant as compared to Delta, “particularly in those who are vaccinated and boosted.”

To some, the new findings represented little more than an attempt by the Biden administration to shift its messaging away from early pledges to pursue a comprehensive response to end the pandemic.

“They’re doing more to manage the narrative on omicron severity than they are to actually manage omicron,” the Columbia University epidemiologist Seth J. Prins wrote on Twitter, expressing the view of some public health officials that greater mitigation measures are necessary.

“We have to look at the big picture, and all of the aspects of the disease,” Boston University public health expert Julia Raifman told Yahoo News. Only 24 percent of Americans have had their booster shot, according to the CDC, meaning they are more susceptible to contracting the coronavirus, especially the hypertransmissible Omicron variant.

Millions of eligible children and adults have not been vaccinated at all, meaning they have far less protection from Omicron unless they retain some natural immunity from a previous COVID-19 illness.

Raifman noted that “Delta was more severe than the initial variant of COVID,” meaning that Omicron’s decreased severity may seem more dramatic than it really is.

Walensky more or less acknowledged the same limitations to the new findings, which came during a briefing of the White House pandemic response team. “The sudden and steep rise in cases due to Omicron,” she cautioned, “is resulting in unprecedented daily cases counts, sickness, absenteeism and strains on our healthcare system.”

Omicron’s primary challenge is not severity but speed: It spreads much faster than Delta, thus compensating in terms of its overall threat to public health for its lower virulence. “The Omicron wave is so spectacularly fast,” the evolutionary biologist Sally Otto told NPR. “I’ve never seen anything spread so quickly in the natural world.”

Still, the data from Southern California were another sign that the setback caused by Omicron may not be as protracted as some may have feared. “The data in this study remain consistent with what we are seeing from Omicron in other countries, including South Africa and the U.K.,” Walensky said, “and provide some understanding of what we can expect over the coming weeks, as cases are predicted to peak in this country.” In both nations, Omicron rose sharply, only to plummet in a little over a month.

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