In a big setback for the city’s recovery, the nation’s largest district will return to all-remote learning.

New York City’s entire public school system will shut on Thursday, signaling that a second wave of the coronavirus has arrived as the city is still struggling to revive from its devastating spring, when it was a global epicenter of the pandemic.

The shutdown was prompted by the city’s reaching a 3 percent test positivity rate over a seven-day rolling average, the most conservative threshold of any big school district in the country. Schools in the nation’s largest system, with 1.1 million students and 1,800 schools, have been open for in-person instruction for just under eight weeks.