"Omicron is spreading at a rate we have not seen with any previous variant," World Health Organization Director-General Dr. Tedros Adhanom Ghebreyesus warned Tuesday.

Omicron has been reported in 77 countries, he said, adding that the new variant is likely in most countries.

Tedros said health officials are "concerned that people are dismissing omicron as mild."

"Even if omicron does cause less severe disease, the sheer number of cases could once again overwhelm unprepared health systems," he said.

-ABC News' Christine Theodorou

British Transport Secretary Grant Schapps announced Tuesday that England will remove all southern African nations from its travel red list.

After the omicron variant was first discovered in South Africa and Botswana in November, several countries around the world, including England and the United States, imposed travel bans on a swath of nations in southern Africa.

The World Health Organization warned that blanket travel bans will not prevent the international spread of omicron, deemed a "variant of concern," and that restrictions place a heavy burden on lives and livelihoods.

The countries of Angola, Botswana, Eswatini, Lesotho, Malawi, Mozambique, Namibia, Nigeria, South Africa, Zambia and Zimbabwe will be taken off England's travel red list on Wednesday at 4 a.m. GMT, according to Schapps, who noted that all current testing measures remain in place.

"As always, we keep all our travel measures under review and we may impose new restrictions should there be a need to do so to protect public health," Schapps wrote on Twitter Tuesday.

Despite the travel bans, the heavily mutated variant has taken a foothold in London. British Health Secretary Sajid Javid told Parliament on Monday that omicron accounts for more than 44% of COVID-19 infections in the U.K. capital and it's expected to become the dominant variant there by Wednesday, overtaking the highly contagious delta variant.

Addressing Parliament again on Tuesday, the health secretary called omicron "a grave threat" and said the "race" to get as many people vaccinated and boosted "is new national mission."

"Scientists have never seen a COVID-19 variant that’s capable of spreading so rapidly," Javid said.

-ABC News' Christine Theodorou

The proportion of new cases of the omicron variant in the United States is steadily growing, according to new data released Tuesday by the Centers for Disease Control and Prevention.

The data show that omicron now accounts for 3% of all new COVID-19 cases nationwide, just over two weeks after it was first detected in the U.S. Last week, omicron was estimated to account for just 0.4% of all new cases nationwide. Epidemiologists, including Dr. John Brownstein, an ABC News contributor, cautioned that these figures are rough estimates.

Currently, the delta variant still accounts for the vast majority -- nearly 97% -- of new cases in the U.S., according to the CDC. More than half the country -- at least 34 states and Washington, D.C. -- have reported cases of the newly discovered variant.

-ABC News' Arielle Mitropoulos

A federal judge in New York has again declined to impose a preliminary injunction on the city’s COVID-19 vaccine mandate for public school employees.

The public school employees have said they have religious objections to the COVID-19 vaccine. The U.S. District Court for the Southern District of New York previously ordered a citywide panel to reconsider their requests for religious accommodation. Most of the plaintiffs were denied a second time and sought the injunction to prevent their termination.

In an order filed Tuesday, Judge Valerie Caproni found New York City's vaccine mandate to be rational, saying: "The Court has no facts before it on which it could conclude that the Citywide Panel’s process was irrational in any way or infected with hostility to religion."

The decision comes a week after the U.S. Supreme Court declined to take up a petition from health care workers who opposed New York state's vaccine mandate on religious grounds. As long as mandates are generally applied and do not single out a particular religion, the courts -- at all levels -- have allowed them to stand.

-ABC News' Aaron Katersky