Massachusetts Gov. Charlie Baker said he's activating 500 more National Guard members to support the state's strained health care system.

Baker had activated 250 National Guard members when students returned to classrooms in the fall to help with school transportation, ABC Boston affiliate WCVB reported.

In Maine, Gov. Janet Mills is activating up to 169 National Guard members to help overrun hospitals, ABC affiliate WMTW reported. Maine hit a hospitalization record Monday with over 400 COVID-19 patients in hospitals, according to WMTW.

New Orleans will reinstate its indoor mask mandate beginning Wednesday as the city gears up for Mardi Gras celebrations.

Masks will be required in restaurants, bars, gyms, entertainment venues and public transportation.

About 64.6% of New Orleans residents are fully vaccinated, according to the city.

People 5 and older must show proof of vaccination or a negative test to access indoor restaurants, bars, entertainment venues, bowling alleys, indoor playgrounds, arcades and similar settings.

Omicron is estimated to account for 98.3% of new cases in the U.S. as of Jan. 8, according to new data released by CDC Tuesday. (These figures are calculated using modeling and may not be exact.)

Delta now accounts for only 1.7% of new cases nationally, forecasters estimate.

In three areas of the country -- the deep South, the Southeast and the New York/New Jersey region -- omicron is estimated to account for more than 99% of new cases.

In early December, omicron was estimated to account for just 0.6% of all new cases.

-ABC News' Arielle Mitropoulos

The White House has been limiting its in-person gatherings to 30 people "over the last several weeks" due to the omicron surge, White House press secretary Jen Psaki said.

“We wear masks in the West Wing at all times, including in your office, unless you're in your office alone,” Psaki said.

-ABC News' Ben Gittleson

Following heated exchanges between Dr. Anthony Fauci and several Republican senators at a Congressional hearing Tuesday, more than 200 prominent science and public health leaders have now penned an open letter voicing their support of the White House chief medical adviser's service and leadership -- and condemning attacks against him. 

"We deplore the personal attacks on Dr. Fauci," the letter says. "The criticism is inaccurate, unscientific, ill-founded in the facts and, increasingly, motivated by partisan politics. It is a distraction from what should be the national focus -- working together to finally overcome a pandemic that is killing about 500,000 people a year."

Signatories include former Senate majority leader Bill Frist, a Republican; Rich Besser, former acting director for the Centers for Disease Control and Prevention; members of President Joe Biden's transition COVID-19 task force; and several Nobel laureates.

The letter comes two days after a Health, Education, Labor and Pensions committee hearing, during which Fauci publicly accused Republican Sen. Rand Paul of fomenting the violent threats and harassment that he and his family have had to contend with during the pandemic's politicized climate.

-ABC News' Sasha Pezenik