U.S. Centers for Disease Control Director Dr. Rochelle Walensky told ABC News the omicron variant is now in at least 16 U.S. states and the agency is "following them closely" but believes the totals are "likely to rise."

Dr. Walensky appeared on This Week Sunday (December 5) morning alongside co-anchor Martha Raddatz to discuss the uncertainty of the new variant and the agency's response to combat it.

"We know it has many mutations, more mutations than prior variants," Walensky said. "Many of those mutations have been associated with more transmissible variants, with evasion of some of our therapeutics, and potentially evasion of some of our immunity, and that's what we're watching really carefully."

Walensky said the delta variant remains the agency's main concern as it continues to be the dominant variant nationwide with thousands of Americans being diagnosed daily.

"We have about 90 to 100,000 cases a day right now in the United States, and 99.9% of them are the delta variant," Walensky said.

Last week, the World Health Organization said the new omicron variant of COVID-19 poses a "very high" global risk as it continues to spread to more countries amid the ongoing pandemic.

“Omicron's very emergence is another reminder that although many of us might think we’re done with Covid-19, it’s not done with us,” Tedros Adhanom Ghebreyesus, director-general of the World Health Organization, told a special session of the World Health Assembly on Monday (November 29) via NBC News.

The omicron variant -- which gets its name from the 15th letter of the Greek alphabet -- was initially detected in Botswana last month and has since been located in South Africa, Germany, Belgium and Hong Kong, before reports of it reaching the U.S.

South African studies showed that the omicron variant was twice as transmissible as delta and Walensky said the public's response to the spread of the variant will be crucial.

"We know from a vaccine standpoint that the more mutations a single variant has, the more immunity you really need to have in order to combat that variant, which is why right now we're really pushing to get more people vaccinated and more people boosted to really boost that immunity in every single individual," Walensky said, adding that she is "hopeful" that current vaccines will at least prevent severe disease and avoid hospitalizations among positive cases.