New York City's school district, the largest in the United States, will halt in-person learning in the latest major restriction as the nation's coronavirus infections soar.
Mayor Bill de Blasio's decision comes as the US death toll from COVID-19 nears a world record of a quarter of a million.
Government officials in dozens of states are also weighing or implementing shutdown measures to stem an unprecedented rate of new infections heading into the winter.
"New York City has reached the 3 per cent testing positivity 7-day average threshold. Unfortunately, this means public school buildings will be closed as of tomorrow, Thursday November 19, out of an abundance of caution," de Blasio wrote on Twitter.
"We must fight back the second wave of COVID-19."
New York City joins other large districts in cities like Boston and Detroit that have cancelled in-person learning.
Within the last week, the Clark County School District, which includes Las Vegas and is the fifth largest in the United States, and Philadelphia's public school system postponed plans to return to in-person learning.
The centre of the US COVID-19 epidemic in the spring, New York City's positive test rate dipped dramatically over the summer but has been gradually rising again.
In other regions, the rate of new positive cases and hospitalisations has skyrocketed.
Nationwide, the number of patients hospitalised with COVID-19 topped 75,000 on Tuesday, setting a new record.
The Midwest has become the new epicentre, reporting almost a half-million cases in the week ending on Monday.
Cuyahoga County, which encompasses Cleveland, on Wednesday ordered residents to stay home "to the greatest extent possible" through December 17 in response to "an unprecedented recent surge of severely ill patients requiring hospitalisation" and "concerns with diminished local hospital bed capacity".
Officials in at least 18 states, representing both sides of the political divide, have issued sweeping new public health mandates this month.
These range from stricter limits on social gatherings and non-essential businesses to requirements for wearing masks in public places.
White House spokeswoman Kayleigh McEnany on Wednesday called the wave of new restrictions an overreach.
"The American people know how to protect their health," she told Fox News in an interview.
"We don't lose our freedom in this country. We make responsible health decisions as individuals."
On Tuesday, the pandemic claimed 1596 US lives, more than on any single day since July 27, contributing to a total of 249,155 confirmed deaths since the pandemic began.
"I'm the most concerned I've been since this pandemic started," Dr. Tom Inglesby, director of Johns Hopkins Center for Health Security, told CNN on Wednesday.
Forty-one US states have reported daily record increases in COVID-19 cases in November, 20 have registered new all-time highs in coronavirus-related deaths from day to day and 26 have reported new peaks in hospitalisations.
In Washington, pressure for a fresh COVID-19 relief bill mounted on Wednesday in the US Congress.