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

A testing company confirmed Wednesday the first known case in Southern California of "flurona," in which someone is infected with COVID-19 and the flu at the same time.

911 COVID Testing told KABC the case was detected at the Getty Center in Brentwood -- a suburban neighborhood in the city of Los Angeles -- in a teenage boy.

The minor was returning from vacation with his family in Cabo San Lucas, Mexico.

Steve Farzam, the testing company's chief operating officer, said the patient has symptoms but was not hospitalized. Since his test, one of his parents has also tested positive for COVID-19, but not the flu.