Some 4,561 U.S. schools either shifted to virtual or closed temporarily for at least one day this week, according to a school opening tracker maintained by Burbio, which aggregates school, government, library and community event information.

That’s a big jump from the few hundred schools where learning was disrupted in early December, though it’s still a small fraction of the 98,000 schools in the U.S., per Burbio.

This week’s number includes 653 Chicago schools that canceled classes today after 73% of Chicago Teachers Union members voted to suspend in-person teaching, Axios’ Monica Eng reports. (The teachers were protesting COVID safety conditions in schools.)