COVID updates: Secretary of Defense Lloyd Austin tests positive

More free COVID-19 testing sites opened in New York City subway stations as several train lines were suspended due to understaffing.

The testing program, which was launched on Monday, originally only included the Grand Central Terminal station and the Times Square-42nd Street subway station -- two of the busiest stations in the city.

However, five more opened Thursday including two in Queens, one in the Bronx, one in Brooklyn and one in Manhattan.

It comes as the Metropolitan Transit Authority announced it is suspending three subway lines due to staffing shortages linked to COVID-19.

"Like everyone in New York, we've been affected by the COVID surge. We're running as much train service as we can with the operators we have available," the MTA said in a statement.

The B and Z subway lines are suspended all day Thursday along with the W line, which was also suspended on Wednesday.

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.