COVID updates: Secretary of Defense Lloyd Austin tests positive

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 appeared on ABC News Live Wednesday and shared his thoughts on the U.S. Centers for Disease Control and Prevention's decision to lower its recommended isolation period to five days for asymptomatic patients.

Fauci also said that while new evidence shows  omicron may be less severe than delta, people should still be cautious of contracting and spreading the virus.

"You can get people safely back out in a five day period so long as they wear a mask, if they are without symptoms. That is the science," he told ABC News Live. "The impact of that is to try and not be in a situation where we essentially have to shut down the entire country."

Fauci also said it would be too premature to be less worried based on evidence that omicron may be less severe.

"We are heartened by the fact that from the South African data, the data from the U.K., and the accumulating data in the United States that it looks like it is less severe," he said. "It's much more transmissible, which is something you need to take seriously."