Secretary of Education Miguel Cardona said Thursday that students should be back in classrooms after winter break despite the surge of the highly transmissible omicron variant.

"If you're fully staffed to provide a safe school environment, students should be in the classroom full-time, every day," he told "Good Morning America's" Whit Johnson.

"We've done this before. We did it before vaccines were available. If you recall, we re-opened schools this year right at the height of the delta variant," Cardona said.

Several school districts across the country said they will require negative tests from students before they can enter classrooms, but Cardona said he doesn't know if that's necessary.

"I do believe all students should have access to testing, but I don't know that it needs to be required," he said.

He continued, "We need to make sure they get into class safely and stay in the classroom."

-ABC News' Molly Nagle

The U.S. Food and Drug Administration (FDA) says "studies are underway" examining why the omicron variant may make rapid COVID tests less effective.

Stephanie Caccomo, a spokeswoman for the FDA, told ABC News the agency will collaborate with the National Health Institute's (NIH) Rapid Acceleration of Diagnostics (RADx) program and select a variety of rapid tests to study.

However, Dr. Bruce Tromberg of the NIH -- who is leading these studies -- said Americans shouldn't be discouraged from using rapid tests.

Tromberg told ABC's Dr. Mark Abdelmalek that rapid tests are "extremely powerful and effective" at identifying people who are infected, with COVID which prevents them from spreading the virus.

"I have confidence that the tests that we have on our shelves can pick up omicron," Tromberg added.

Testing companies also stressed that their tests still work to detect omicron, with Abbott saying Tuesday that the company has tested the popular BinaxNOW rapid test and found "equivalent sensitivity" compared to prior variants.

-ABC News' Sony Salzman

Hospitals across England were setting up "Nightingale surge hubs," temporary structures capable of handling 100 patients, as they prepared for a potential wave of omicron patients.

The United Kingdom's National Health Service announced the plan on Thursday, a day after health officials recorded a new daily high for COVD-19 infections.

"Given the high level of COVID-19 infections and increasing hospital admissions, the NHS is now on a war footing," NHS National Medical Director Professor Stephen Powis said in a statement on Thursday.

The NHS said it would start with hubs at eight hospitals around the country. Health Secretary Sajid Javid said in a statement that an additional 4,000 beds could be added if necessary.

“We do not yet know exactly how many of those who catch the virus will need hospital treatment, but given the number of infections we cannot wait to find out before we act and so work is beginning from today to ensure these facilities are in place," Powis said.

The United Kingdom, Italy and Greece broke records on Wednesday for daily new COVID-19 cases, according to government health officials.

The Greek government reported a record increase for new cases for a third consecutive day with 28,828 new cases and 72 COVID-19-related deaths.

About 64.3% of the population is fully vaccinated and 26.5% have received a booster or third dose, according to the European Centre for Disease Prevention and Control.

Health Minister Thanos Plevris announced on Wednesday that new restrictions will go into effect on Jan. 3, but Greece's Kathimerini newspaper reported that the government has decided to put them into effect at 6 p.m. local time on Thursday.

Italian health authorities recorded another record increase on Wednesday with 98,0303 new COVID-19 cases and 148 deaths.

Semi-official ANSA reported Wednesday's positivity rate is 9.5% and there are 1,185 patients in ICUs. About 51.7% of Italy's residents are fully vaccinated and 24.9% have received a booster or 3rd dose, per the ECDC.

UK health officials recorded 183,037 new cases and 57 deaths. The record-setting new case number included five days of data from Northern Ireland, which last published data on Dec. 24, according to health officials.

About 82.3% of adults in the UK have received two vaccine doses and 57.5% have received a booster, according to government data.

-ABC News' Christine Theodorou

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