Officials in Chicago canceled all public school classes on Thursday amid discussions about classroom safety with the city's teachers.

Classes had been canceled on Wednesday after a majority of the Chicago Teachers Union's membership voted in favor of remote learning during a surge in COVID-19 cases. School officials called their action an illegal strike.

"In a time of crisis related to this pandemic, the worst possible thing we can do is abandon the science and data," Mayor Lori E. Lightfoot said on Twitter. "If you care about our students and families as we do, we will not relent. We are standing firm and fighting to get our kids back to in-person learning."

Teachers were locked out of their remote classrooms on Wednesday, according to the union. Union leaders asked members to again try to log in on Thursday, urging them post photos on social media.

The union on Wednesday filed an unfair labor practice charge against the Chicago Board of Education.

"We have rights to safety and we’ve been at the bargaining table for 20 months to secure those rights," Jesse Sharkey, the union's president, said in a statement.

Chicago Public Schools are among the largest in the country, with about 340,000 students in 636 schools.

The Transportation Security Administration reported 3,037 current COVID-19 infections on Wednesday.

The agency's infections have increased by about 16% in two days, according to TSA data.

The agency, which employs about 60,000, said it's had a cumulative 15,191 COVID-19 cases. The agency said 12,154 employees have recovered and 33 have died.

ABC News' Sam Sweeney

The director of the Centers for Disease Control and Prevention has given the final go-ahead for children ages 12 to 15 to get Pfizer's COVID-19 booster.

"It is critical that we protect our children and teens from COVID-19 infection and the complications of severe disease," CDC Director Rochelle Walensky said in a statement endorsing the CDC advisory panel's recommendation to expand booster eligibility.

The CDC recommends that adolescents between the ages of 12 and 17 get a Pfizer booster five months after their second dose.

-ABC News' Cheyenne Haslett

COVID-19 cases have been reported on every cruise ship operating in U.S. waters with passengers, according to the U.S. Centers for Disease Control and Prevention.

More than 90 ships are currently being investigated by the CDC or have been investigated and are still being observed by the agency.

In order to meet the threshold for a CDC investigation, a ship must report COVID-19 cases in more than 0.10% of passengers or have a single crew member test positive in the previous seven days.

The only ships in U.S. waters that have not met this criteria, according to the CDC, only have crew members on board.

-ABC News' Mina Kaji

Dr. Rochelle Walensky, director of the U.S. Centers for Disease Control and Prevention, is standing by her agency's new quarantine and isolation guidance for COVID-19, which the nation’s largest association of physicians has criticized as "confusing."

The CDC updated its guidelines on Dec. 27, saying asymptomatic people who test positive for COVID-19 should self-isolate for five days rather than 10. In a scathing statement released Wednesday night, the American Medical Association (AMA) said the new recommendations "are risking further spread of the virus."

"The American people should be able to count on the Centers for Disease Control and Prevention (CDC) for timely, accurate, clear guidance to protect themselves, their loved ones, and their communities. Instead, the new recommendations on quarantine and isolation are not only confusing, but are risking further spread of the virus," the AMA's president, Dr. Gerald E. Harmon, said in the statement. "With hundreds of thousands of new cases daily and more than a million positive reported cases on January 3, tens of thousands -- potentially hundreds of thousands of people -- could return to work and school infectious if they follow the CDC’s new guidance on ending isolation after five days without a negative test."

But Walensky defended the new guidance, telling ABC News' Cecelia Vega in an interview Friday on "Good Morning America" that the CDC "make[s] these recommendations in the context of science, in the context of ongoing epidemiology and in the context of what is feasible in collaboration with our public health and local and state public health partners."

"I've read the AMA statement and I have deep respect for the AMA," Walenskey said, "but I will also say we've heard a lot of support for ongoing guidance from public health partners, from other clinical and laboratory partners as well."

Walensky referenced "dozens of studies" that have shown a COVID-19 patient is most infectious in the one to two days before symptoms and the two to three days after symptoms.

"So by five days after your symptoms, the vast majority of your contagiousness is really behind you," she said. "And what we say at day five then is, are your symptoms gone? Are you feeling better? Is your cough gone, sore throat gone? And if so, then it is safe to go out if you are wearing a mask all the time. And that means not going out to restaurants, not going out to gyms, not going out and visiting grandma, but really conscientiously wearing your mask for those last five days."

Walensky noted that people may choose to take an antigen or at-home test at day five, if they have access to one. But it's not required.

"If that test is positive, you should stay home for those extra five days. But if that test is negative you should go out and continue to wear your mask," she said. "You know, these tests are actually not authorized for the purpose of evaluating contagiousness."