The requirements come at a time when new coronavirus infections are accelerating nationwide, especially in northern states, as colder weather has encouraged more mingling and socializing indoors.
Over the last week, the country has averaged more than 120,000 new infections a day, up 64% from the prior week, according to a Reuters tally.
Deaths, which lag infections, have averaged 1,300 a day over the last seven days, up from an average of 800 a day a week ago, according to Reuters data.
The Delta variant still accounts for 99.9% of new COVID cases in the United States, CDC Director Dr. Rochelle Walensky told ABC News on Sunday.
Omicron, first detected last month in southern Africa, has spread around the globe and shows signs of being more contagious than the Delta variant.
A total of several dozen Omicron cases have been found in 17 out of 50 U.S. states: California, Colorado, Connecticut, Georgia, Hawaii, Maryland, Massachusetts, Louisiana, Minnesota, Missouri, Nebraska, New Jersey, New York, Pennsylvania, Utah, Washington and Wisconsin, according to a Reuters tally.
Louisiana has also reported a probable Omicron case in a crew member on a cruise ship that disembarked in New Orleans over the weekend. At least 17 COVID-19 cases were detected on the ship and more testing is underway, state health officials said.
Several Wall Street banks headquartered in New York, including Goldman Sachs Group Inc, Morgan Stanley and Citigroup, already require vaccines for anyone coming into their offices. JPMorgan Chase & Co, the largest U.S. bank, has so far allowed unvaccinated employees to come to work in offices if they submit to twice-weekly COVID-19 tests.
Alphabet Inc's Google and Meta's Facebook, which also have operations in New York City, also already require all U.S. employees to be vaccinated to enter buildings.
A nationwide vaccine mandate issued earlier this year by President Joe Biden for companies with 100 workers or more has been tied up in litigation. In November, a US appeals court upheld its decision to put on hold the order.
De Blasio, noting that the city has already issued mandates covering several other sets of municipal workers, expressed confidence that his latest order would withstand legal scrutiny.
"We are confident because it's universal," he said on MSNBC.