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

More than 300 children with COVID-19 are in Ohio hospitals, including Dayton Children's Hospital, where workers are seeing a significant increase in pediatric COVID-19-related hospitalizations.

Dr. Vipul Patel, chief of pediatric intensive care at Dayton Children's, told ABC News the ICU is now busier than at any other point in the pandemic.

COVID-19 is only exacerbating previously existing health issues for many children, Patel explained, adding that many parents are shocked to see their children become so sick, and some families have even expressed regret for not vaccinating their kids. Nationwide, about 35% of eligible children (ages 5 to 17) are fully vaccinated, according to federal data.

Dayton Children's respiratory therapist Hillary O’Neil said it's been particularly difficult to see children who are too young to understand what is happening sick and scared.

“You can see it in the faces of kids that can’t talk -- their eyes get really big and they, we watch them struggle to breathe,” O’Neil said. “Then on top of that we watch their parents struggle to watch their child, and that is sometimes just as hard as watching the kids.”

Jackie Kerby, whose baby, Enaeshya, is hospitalized with COVID-19, told ABC News, "She’s getting these fevers in the night, and they’re not coming down. … I am terribly scared."

Across the U.S. more than 5,000 children are currently hospitalized with a confirmed or suspected case of COVID-19, according to federal data. On average, hospital admissions among children have quadrupled over the last month.

-ABC News' Arielle Mitropoulos, Kayna Whitworth