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

Spain has reported more COVID-19 reinfections in a recent span of two weeks than it has during the rest of the pandemic, according to the latest data from a Spanish public health research institute.

Data from the Carlos III Health Institute shows there were 20,890 repeat infections reported in Spain from Dec. 22, 2021, to Jan. 5, 2022. That figure tops the 17,140 reinfections reported in the European country from the start of the pandemic in March 2020 to Dec. 22, 2021. The data includes confirmed, probable and possible cases.

Alfredo Corell, an immunology professor at Spain's University of Valladolid, told Spanish news site NIUS that the rising number of reinfections were due to the new and highly transmissible omicron variant.

"Prior to this variant, reinfections were anecdotal at the global level," Corell told NIUS. "Omicron has changed this paradigm."

In southern Africa, where the variant was first identified in November, preliminary research suggests that omicron is three times more likely to cause reinfections compared to other known variants of the virus, including the highly contagious delta. However, symptoms of reinfected individuals appear to be mild, according to Anne von Gottberg, a microbiologist at South Africa's National Institute for Communicable Diseases who is studying omicron.

"Previous infection used to protect against Delta," von Gottberg said during a press briefing on Dec. 2. "But now, with Omicron, that doesn't seem to be the case."