By Peter Szekely and Brendan O'Brien

NEW YORK (Reuters) -New York City declared on Monday that all private-sector employers must implement COVID-19 vaccine mandates for their workers, as the highly transmissible Omicron variant has spread to at least one-third of U.S. states.

The biggest U.S. city set a December 27 deadline for all 184,000 businesses within its limits to make their employees show proof that they have been vaccinated.

In addition, children 5 to 11 years old must get at least one vaccine dose by Dec. 14 to enter restaurants and to participate in extracurricular school activities, such as sports, band, orchestra and dance, Mayor Bill de Blasio said.

"Vaccination is the way out of this pandemic, and these are bold, first-in-the-nation measures to encourage New Yorkers to keep themselves and their communities safe,” de Blasio, who leaves office next month, said in a statement.

De Blasio's successor, Eric Adams, “will evaluate this mandate and other COVID strategies when he is in office and make determinations based on science, efficacy and the advice of health professionals,” said his spokesperson Evan Thies.

About 27 percent of children ages five to 12 have gotten at least one dose and 15 percent are fully vaccinated, according to the city's website.

The Greater New York Chamber of Commerce said it supported the expanded mandate.

The requirements come at a time when new coronavirus infections are accelerating nationwide https://tmsnrt.rs/2WTOZDR, 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 percent 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 18 out of 50 U.S. states: California, Colorado, Connecticut, Georgia, Hawaii, Maryland, Massachusetts, Louisiana, Minnesota, Mississippi, 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 U.S. 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.

(Reporting by Peter Szekely in New York and Brendan O'Brien in Chicago; Additional reporting by Elizabeth Dilts in New York, Susan Heavey in Washington and Barbara Goldberg in Maplewood, New Jersey; Editing by Doina Chiacu and Lisa Shumaker)