Four Smithsonian locations in Washington, D.C., have closed due to an increase in COVID-19 cases over the last few days.

The four museums -- the National Museum of African Art, the National Postal Museum, the Anacostia Community Museum and the National Museum of Asian Art -- are expected to reopen Jan. 3.

The Smithsonian said the cases and quarantine periods impacted "essential and operational staff," so these closures "will allow the Smithsonian to reallocate staff and keep all other museums open for the remainder of the week.”

The Smithsonian closed in March 2020 due to COVID-19 and loccations started to reopen in May 2021.

-ABC News' Beatrice Peterson

All public school students and staff in Washington, D.C., must test when they return to the classroom after winter break.

Free rapid tests will be provided by the city to the over 90,000 students in the largest data collection D.C. has done since the pandemic began, Mayor Muriel Bowser said.

The tests will be distributed on Jan. 3 and Jan. 4. Families are asked to upload results to the city by Jan. 4 at 4 p.m.

“Any student that does not have their results loaded by Jan. 4, will not be allowed to attend school on Jan. 5," school chancellor Lewis Ferebee told reporters.

Bowser told reporters, "We expect that we're going to be in this winter surge for a few more weeks, so throughout January we're going to have to maintain vigilance."

-ABC News' Beatrice Peterson

Dr. Anthony Fauci strongly recommended against large New Year's Eve parties at Wednesday's White House briefing.

"Should you change or cancel your plans? If your plans are to go to a 40-50 person New Year's Eve party with all the bells and whistles and everybody hugging and kissing and wishing each other a happy new year, I would strongly recommend that this year, we do not do that," Fauci said.

Fauci said "all indications point to a lesser severity of omicron versus delta." But he warned, "we should not become complacent since our hospital system could still be stressed in certain areas."

White House COVID-19 coordinator Jeff Zients said, "We have directly helped more than 30 states and territories by deploying over 2,100 federal personnel and thousands of ambulances, ventilators and other critical supplies."

Supplies include gloves, masks, respirators and face shields, he said.

He added, "More than 13,000 National Guard members have been activated in 48 states to support the COVID response, from vaccinations, to testing, to clinical care."

At the briefing, CDC Director Rochelle Walensky defended the agency's shortened isolation guidelines.

"We do know the vast majority of viral transmission happens in those first five days, somewhere in the 85 to 90% range. So if a person can isolate for the first five days they absolutely should," she said. "We also don't know that antigen tests give a good indication of transmissibility at this stage of infection. On the other hand, we know that after five days people are much less likely to transmit the virus and that masking further reduces that risk. And this is why people need to mask for five days after the five days of isolation."

After five days, asymptomatic people with COVID-19 can leave isolation, but must wear masks around others, according to the new guidelines.

-ABC News' Cheyenne Haslett

Omicron and delta are twin threats driving up cases to record numbers and leading to spikes in hospitalizations and deaths, WHO Director-General Dr. Tedros Adhanom Ghebreyesus said at a news conference Wednesday.

"I am highly concerned that omicron being more transmissible, circulating at the same time as delta, is leading to a tsunami of cases," Tedros said.

As the pandemic drags on, it's possible that new variants could become resistant to current vaccines, necessitating vaccine adaptations, he warned.

He added, "While 2021 has been hard, I ask everyone to make a New Year’s resolution to get behind the campaign to vaccinate 70% by the middle of 2022."

-ABC News' Christine Theodorou

Dr. Anthony Fauci said a COVID vaccine requirement for domestic air travel should be "seriously" considered.

"If you're talking about requiring vaccination to get on a plane domestically, that is just another one of the requirements that I think is reasonable to consider," he said Monday on MSNBC's "Morning Joe."

"When you make vaccination a requirement, that's another incentive to get more people vaccinated. If you want to do that with domestic flights, I think that's something that seriously should be considered," he said.

This is not the first time that Fauci has argued for vaccine mandates domestic flights.

On Sunday, Fauci told ABC's Jon Karl that "anything that could get people more vaccinated would be welcome."

-ABC News' Arielle Mitropoulos