The World Health Organization on Thursday pushed back against the consistent chatter that the ultra-transmissible omicron coronavirus is "mild," noting that the variant is causing a "tsunami of cases" that is "overwhelming health systems around the world."

"While omicron does appear to be less severe compared to delta—especially in those vaccinated—it does not mean it should be categorized as 'mild,'" WHO Director-General Dr. Tedros Adhanom Ghebreyesus said in a press briefing Thursday. "Just like previous variants, omicron is hospitalizing people, and it is killing people."

The warning comes as the US is still experiencing a vertical rise in cases and hospitalizations from the quick-spreading variant. In the week ending on January 1, omicron was estimated to account for 95 percent of all cases in the US, according to the latest analysis by the US Centers for Disease Control and Prevention. The delta variant, which was making up over 99 percent of US cases as recently as the week ending on December 4, has now been relegated to just 5 percent of cases.

With omicron's speedy rise to dominance comes a towering wave of cases. The US logged more than 700,000 new cases Wednesday after adding over 1 million on Monday. The seven-day average for new daily cases is over 585,000, which is a 247 percent increase from two weeks ago.

Hospitalizations are also rising sharply in the US, despite consistent reports that a smaller proportion of omicron cases lead to severe disease and hospitalizations. In addition, reports from hospitals in the US, the UK, and South Africa suggest that those who do end up in the hospital with omicron are faring better, requiring less supplemental oxygen, less intensive care, and less ventilation. But that doesn't mean that people across the board are being spared from the worst cases of COVID-19.

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“We should not be complacent”

In the US right now, the daily average number of people hospitalized with COVID-19 is up to 110,334, a 58 percent increase from two weeks ago. And hospitalizations still appear to be rising.

Of particular concern are the rising hospitalizations among children. More than 4,000 children are currently in the hospital with COVID-19, an all-time high in the pandemic, according to tracking by The Washington Post. The current number of hospitalized children is nearly double that from two weeks ago, when fewer than 2,000 were hospitalized. Several doctors and health experts have anecdotally reported seeing higher levels of COVID-related croup and bronchiolitis in children.

In an interview Sunday on CBS's Face the Nation, former commissioner of the Food and Drug Administration Scott Gottlieb said that omicron appears to cause more disease in the upper airway rather than deep in the lung, where severe disease can take hold. That may be a problem for "very young children—toddlers—who have trouble with upper airway infections, and you're in fact seeing more croup-like infections and bronchiolitis in New York City among children."

In a White House press briefing Wednesday, top infectious disease expert Anthony Fauci highlighted the accumulating evidence finding less severe disease in the omicron wave. Though his tone was far less dire than that of Dr. Tedros, he still added a note of caution about omicron's threat.

"We should not be complacent," Dr. Fauci said. "The increased transmissibility... of omicron might be overridden by the sheer volume of the number of cases." Those cases "may be of reduced severity but could still stress our hospital system because a certain proportion of a large volume of cases, no matter what, are going to be severe."