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

Uganda reopened its schools to students on Monday after nearly two years, ending the world's longest school closure due to the coronavirus pandemic.

Schools across the East African nation of 44 million people have been fully or partially closed since March 2020, when the pandemic began. The closures affected more than 10 million learners, according to data from the United Nations Educational, Scientific and Cultural Organization (UNESCO).

Save the Children welcomed the reopening of Uganda's schools but warned that "lost learning may lead to high dropout rates in the coming weeks without urgent action."

The London-based charity revealed in a report last November that up to one in five children in low-income countries, including Uganda, had dropped out of school due to rising poverty, child marriage and child labor, which have been exacerbated by the pandemic. In a statement Monday, Save the Children warned of "a 'second wave' of dropouts as returning students who have fallen behind in their learning fear they have no chance of catching up."

To tackle the potential crisis in Uganda, Save the Children has launched "Catch-up Clubs," which assess children and teaches them at the required level to help them regain literacy and other learning, with child protection support and cash assistance for families struggling to send them to school.

"As schools begin to reopen across the country, it is critical that all girls and boys have access to the support they need to successfully return to the classroom," Edison Nsubuga, head of education at Save the Children in Uganda, said in a statement Monday. "Many children have fallen behind in school as a result of the COVID-19 pandemic. Children who are behind in their learning are less likely to unlock their potential as adults. However, when children receive the learning boost they need and have access to quality education, they can reach their full potential."