Chicago’s public schools shut their doors on Wednesday and began shifting to remote learning following a vote by the city’s teachers’ union to do so out of concern over COVID-19.

Members of the Chicago Teachers Union (CTU) voted to move classes online out of fear that rising cases of COVID-19 will jeopardize the health of students and education staff as the new semester begins. In a press release announcing the move, the CTU laid blame on the administration of Mayor Lori Lightfoot and the Chicago Public Schools (CPS) leadership for failing to ensure a safe return to classes.

“Let us be clear. The educators of this city want to be in their classrooms with their students. We believe that our city’s classrooms are where our students should be,” said the union, which is made up of 25,000 members.

“Regrettably, the Mayor and her CPS leadership have put the safety and vibrancy of our students and their educators in jeopardy."

Tensions between the mayor and the teachers union were palpable ahead of the CTU’s final vote to move towards remote learning this semester.

Lightfoot, a Democrat, argued at the start of this week that it was imperative that children stayed in schools, insisting that they did not constitute a source of "significant spread" in Chicago.

Ahead of the CTU’s vote on Wednesday, Lightfoot slammed the teachers union for engaging in what she said was an "illegal work stoppage" and called the move to remote learning "disruptive...to individual parents who have to work, who can't afford the luxury of staying home" with their kids. Later, the mayor took to Twitter to assure parents that she would be fighting back to return children to the classroom.

“I want to assure you that I am doing everything in my power to keep our students in school, where they belong, learning,” tweeted Lightfoot.

"I'm urging teachers. Show up to your schools. Your kids need you,” she continued.

Chicago, which has over 345,000 students attending its public schools, has become the largest city to close its schools as a new semester resumes. Smaller cities, including Atlanta and Milwaukee, have shifted to a limited period of online learning as have a number of universities across the United States over COVID fears.

On the other side of the argument, President Joe Biden weighed in by saying that the science shows it is safe for children to attend classrooms despite COVID-19.

“We have no reason to think at this point that Omicron is worse for children than previous variants. We know that our kids can be safe when in school by the way. That’s why I believe schools should remain open. They have what they need,” Biden said on Tuesday.

New York City, which hosts the nation’s largest public school district, has chosen to remain open. Former Mayor Bill de Blasio and his successor Eric Adams have echoed Lightfoot by insisting that schools are safe for attendance and closing them was not on the cards. Adams, who was sworn in on New Year's Day, has been insistent that school is the best place to be for children amidst calls from the city teachers to move to remote learning.

“I’m not going to allow the hysteria to prevent the future of my children receiving a quality education,” Adams told CNN’s Brianna Keilar.