New York's public schools will close temporarily to combat a rise in coronavirus cases, its mayor announced Wednesday, dealing a blow to the city's recovery.
Bill de Blasio said the schools would shut from Thursday "out an abundance of caution" after the city recorded a seven-day average positivity rate of three percent.
"We must fight back the second wave of Covid-19," he wrote on Twitter.
The three percent threshold had been agreed between the city government and teaching unions when New York's schools began reopening in September.
New York City is the largest school district in the United States with 1.1 million students.
It has been the only major city in the United States to commit to offering in-person classes this fall as part of a hybrid system that included online learning.
Many cities, such as Chicago, Houston, Los Angeles, Philadelphia and Miami, opted instead for the virtual model.
New York's closures are controversial with testing showing that schools are not the source of soaring cases. The positivity rate in schools is just 0.19 percent, Richard Carranza, the head of New York public schools, told principals in an email.
De Blasio's announcement will see hundreds of thousands of children return to remote learning and leave parents scrambling to make last-minute childcare arrangements.
The mayor had warned of an imminent closure since the end of last week, with parents enduring a daily nervous wait for the latest positivity rate.
They appeared to have been granted another reprieve Wednesday when New York State Governor Andrew Cuomo told reporters that the city's rolling seven-day average was only 2.50 percent.
Cuomo said New York City could be declared an "orange zone" once it touched three percent, also triggering the shutdown of indoor dining and gyms.
As Cuomo briefed reporters, de Blasio tweeted that in fact the three-percent threshold had been met. He did not immediately provide information on non-essential activities such as gyms.
The pair, both Democrats, have a difficult relationship and are widely known to dislike each other.
New York's 1,800 public schools were first closed on March 15 when the city became the early epicenter of America's virus outbreak.
They were initially shuttered until April 20 before being closed until the end of the school year as the pandemic engulfed the US's largest city. They began opening again in September.
More than 24,000 people have died from Covid-19 in New York City, the majority in the spring and early summer, according to city government statistics.
New York's positivity rate is currently much lower than many other areas of the US, which is currently seeing a surge of cases.