About 75% of people in the U.K. experiencing new cold-like symptoms actually have symptomatic COVID-19, a new study suggests.

Health science company ZOE, which has been conducting the ongoing study with King's College London, said the figure is based on a decline in the number of non-COVID "colds" and a sustained increase in symptomatic COVID cases.

Dr Claire Steves, scientist on the ZOE COVID Study app and Reader at King’s College London, said she wants the U.K. government's list of symptoms to be updated to include headache, runny nose and sore throat.

"The fact that 75% of new cold-like symptoms are COVID, and the classic symptoms are much less common, means the government advice needs to be urgently updated," she said in a release Thursday.

On Wednesday, the U.K. recorded a record-high 183,037 cases of COVID-19, more than double the figure reported at this time last year.

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