Omicron updates: COVID outbreak reported on cruise ship docking in New Orleans

About 100 million Americans remain completely unvaccinated, according to federal data. Nearly 80 million of those people are over the age of 5 and therefore eligible to get the shot.

Since the beginning of November, hospital admissions have jumped by 20%, while emergency department visits have increased by 27%, according to federal data.

Minnesota and Michigan currently hold the country's highest case rate, followed by Wisconsin, North Dakota, New Hampshire, Arizona, New York, Pennsylvania and Ohio. Puerto Rico, Florida and Louisiana currently have the nation's lowest infection rate, according to federal data.

-ABC News' Arielle Mitropoulos

Greek Prime Minister Kyriakos Mitsotakis said Tuesday that vaccines for people 60 and older will be required.

Appointments for the first dose must be booked by Jan. 19 and residents who don't comply will be fined.

There are 664 people currently hospitalized with COVID-19 in Greece and the average age of those patients is 64.

The nation's vaccination rate stands at 62%.

Greece has not yet detected a case of the omicron variant but Mitsotakis said it's just a matter of time.

-ABC News' Christine Theodorou

World Health Organization Director-General Dr. Tedros Adhanom Ghebreyesus said Tuesday that "we should not be surprised" by the emergence of omicron, because "this is what viruses do."

"The longer we allow the pandemic to drag on -- by failing to address vaccine inequity, or to implement public health and social measures in a tailored and consistent way -- the more opportunity we give this virus to mutate in ways we cannot predict or prevent," Tedros said.

He continued, "We still have more questions than answers about the effect of Omicron on transmission, severity of disease, and the effectiveness of tests, therapeutics and vaccines. The global response must be calm, coordinated and coherent. At the same time, we must not forget that we are already dealing with a highly transmissible, dangerous variant – the Delta variant, which accounts for almost all cases globally."

The FDA in a new statement said it's working as quickly as possible to evaluate the potential impact of omicron on the currently available diagnostics, therapeutics and vaccines.

"Historically, the work to obtain the genetic information and patient samples for variants and then perform the testing needed to evaluate their impact takes time,' the FDA said. "However, we expect the vast majority of this work to be completed in the coming weeks."

The FDA stressed that vaccines, boosters and masks are the best ways to stay protected.

-ABC News' Eric M. Strauss