CHICAGO — The Chicago Teachers Union said Sunday that its members voted to defy an order to return to the classroom before they are vaccinated against the coronavirus, setting up a showdown with district officials who have said such a move would amount to an illegal strike.

The nation's third-largest school district wanted roughly 10,000 K-8 teachers and other staffers to return to school on Monday to get ready to welcome back roughly 70,000 students for part-time in-school classes starting Feb. 1. No return date has been set for high school students.

The teachers union, though, opposed the plan over concern for the health of its members and called on them to continue teaching from home in defiance of the district's plan. 

The roughly 355,000-student district, which turned to full-time online instruction last March because of the pandemic, has gradually welcomed students back. The parents of thousands of pre-K and special education students chose this month to resume in-person learning, and teachers who didn't return to their classrooms were punished.

The union's collective bargaining agreement, which was approved after a 2019 strike, prohibits its roughly 25,000 members from striking and bars district officials from locking them out. District officials have said a union vote to disobey the order to return to schools on Monday would violate the contract. 

Union officials, though, say returning to in-person instruction before its members are vaccinated would put them at greater risk of contracting the virus. They argue that if the district tries to punish teachers for staying home Monday, then the district would be responsible for a work stoppage.

The district on Friday said it would begin vaccinating teachers and staff starting in mid-February and that the process would take months. 

A union lawyer told the Chicago Sun-Times that the union wants to let teachers and other staffers continue working from home until they are vaccinated, with individuals returning to school after receiving the first of their two vaccine shots. 

The vote comes at a time of great uncertainty in the U.S. about how and when schools should resume in-person instruction.

President Joe Biden has pledged  to have a majority of schools reopened within his first 100 days in office. He is promising new federal guidelines on school opening decisions, and a "large-scale" Education Department effort to identify and share the best ways to teach during a pandemic.