The shift reflects the spread of the omicron variant.
As the COVID-19 pandemic has swept the globe, more than 5.3 million people have died from the disease worldwide, including over 816,000 Americans, according to real-time data compiled by Johns Hopkins University's Center for Systems Science and Engineering.
About 61.7% of the population in the United States is fully vaccinated against COVID-19, according to data from the Centers for Disease Control and Prevention.
The University of Chicago said students and employees must get a booster shot and have until the end of January to give proof or get an exemption.
The university hopes to fully return to in-person classes on Jan. 3.
Other universities like New York University and Notre Dame are also requiring boosters.
-ABC News' Oren Oppenheim
Queen Elizabeth II has changed her Christmas plans and canceled her traditional family gathering at Sandringham, her Norfolk estate. A royal source told ABC News this was a personal decision and the queen was following a precautionary approach as COVID-19 cases sharply rise in the United Kingdom.
The queen instead will remain at Windsor Castle where her family is expected to visit.
-ABC News' Zoe Magee
WHO DG recommends canceling or delaying holiday events
World Health Organization Director-General Dr. Tedros Adhanom Ghebreyesus on Monday recommended delaying or canceling holiday events.
"There is now consistent evidence that omicron is spreading significantly faster than the delta variant. And it's more likely that people who have been vaccinated or have recovered from COVID-19 could be infected or re-infected," he said.
While everyone wants a return to normal, he said, the fastest way to get there is to make difficult decisions to protect ourselves and others.
"In some cases, that will mean canceling or delaying events," he said. "But an event canceled is better than a life canceled."
-ABC News' Christine Theodorou
Washington, D.C., Mayor Muriel Bowser has declared a state of emergency and is reinstating an indoor mask mandate from Dec. 21 to Jan. 31, 2022.
D.C. is also expanding testing, including offering free at-home rapid tests, the mayor said.
