The city of Chicago called off school after the Chicago Teachers Union voted overwhelmingly Tuesday night to stay home, insisting on a return to remote learning during a winter surge in coronavirus infections. On Wednesday evening, the city announced schools would be closed a second day.

“The only thing we can control is whether we go into the buildings,” Jesse Sharkey, the union president, said Wednesday. “Right now, going into schools puts us at risk, puts students and families at risk, of contracting the coronavirus.”

He said teachers would not come back before Jan. 18 unless the surge in cases subsides or the union reaches an agreement with the city, chiefly on additional virus testing.

The union’s chief demand was for an effective coronavirus testing program for students, a process that has gone far more smoothly in other cities. In Chicago, the district encouraged students to take coronavirus tests near the end of the winter break but thousands of those tests were deemed invalid.