Omicron is now the dominant variant of the coronavirus in the US, accounting for 73 percent of new infections last week, the Centers for Disease Control and Prevention said Monday.

The CDC numbers showed nearly a sixfold increase in Omicron's share of infections in one week. As recently as the end of November, more than 99.5 percent of coronaviruses were of the Delta variant, according to CDC data.

Breakthrough infections also are rising in the US, where 61 percent of Americans are fully vaccinated and 30 percent have gotten booster shots, according to the CDC.

The White House said Monday that President Joe Biden does not plan on "locking the country down" in response to a surge in coronavirus cases.

Biden will "announce additional steps" in the fight against COVID-19 in a speech he is to deliver on Tuesday, but he won't necessarily talk about additional restrictions, White House press secretary Jen Psaki said.

"This is not a speech about shutting the country down," Psaki said. "This is a speech outlining and being direct and clear with the American people about the benefits of being vaccinated, the steps we're going to take to increase access and to increase testing, and the risks posed to unvaccinated individuals."

In a statement posted to the White House website Monday evening, Psaki said that "a mid-level staff member, who does not regularly have contact with the President, received a positive result for a COVID-19 test".

She said that a staff member had spent approximately 30 minutes around the president on Air Force One, en route from Orangeburg, South Carolina, to Philadelphia.

"This staff member is fully vaccinated and boosted, and tested negative prior to boarding Air Force One, as is required for everyone traveling with the President," the statement said. "This staff member did not begin to experience symptoms until Sunday and was tested on Monday."

Psaki said that the president is tested regularly and received an antigen test Sunday that was negative. He received a PCR (polymerase chain reaction) test Monday that also was negative, she said, and he will be tested again Wednesday.

In New York City, coronavirus cases increased by 60 percent last week. Statewide, New York has set records for the most cases reported in one day since the pandemic started, for three consecutive days.

An estimated 90 percent of new infections in the New York area, the Southeast, the industrial Midwest and the Pacific Northwest are caused by the Omicron variant, The Associated Press reported.

Many Broadway productions canceled performances as cases or crew members tested positive for the coronavirus. The production of Hamilton on Monday announced the cancellation of all performances until after Christmas due to breakthrough coronavirus infections.

Washington DC Mayor Muriel Bowser on Monday declared a state of emergency and reinstated an indoor mask mandate until the end of January. She also required local government workers to get vaccinated, including a booster shot.

Maryland Governor Larry Hogan said he received a positive coronavirus rapid test Monday.

"I have been vaccinated and boosted, and I am feeling fine at the moment," Hogan tweeted. He also urged Marylanders to get vaccinated and boosted.

Hogan still was waiting for confirmation from a more sensitive PCR test from a lab, according to his office.

Senators Elizabeth Warren of Massachusetts and Cory Booker of New Jersey announced separately on Sunday that they had tested positive for the coronavirus.

Both senators were vaccinated and had received a booster shot, and they said they were experiencing mild symptoms.

Moderna announced Monday that preliminary data suggest its half-dose, third booster shot will be effective against the Omicron variant.

Moderna's results show that blood from 20 people who received the 50-microgram Moderna booster had 37 times the number of neutralizing antibodies as compared with blood from the same number of people who received only two shots.

A group that received a third shot of the higher, 100-microgram dose saw an 83-fold jump in neutralizing antibodies against Omicron, the company said.

Moderna's COVID-19 vaccine booster is currently authorized for emergency use for people age 18 and older in the US and administered as a half-dose at 50 micrograms.

The company had reduced the dose of its booster to half the dose of the original two shots to limit side effects such as fever, muscle aches and fatigue.

It's now up to US public health authorities to consider whether Moderna's booster should be given as a larger dose, Moderna President Stephen Hoge said Monday.

Without a booster, Moderna's COVID-19 vaccine was found to be far less effective against the Omicron variant, generating low neutralizing antibodies. Although the vaccines may not prevent infection from the variant, health officials have said they prevent severe illness.

The data have not been published or peer reviewed.

The company also said it will continue to develop an Omicron-specific booster candidate that should enter clinical trials early next year.