The city's daily COVID positivity rate is at 2.7%, with the seven-day average at 1.92%.
There are 81 patients hospitalized daily with 532 reported cases, just below the city's threshold of 550.
"That alone is not a number that would overwhelm us, but the growth is what worries me," de Blasio said. "We cannot allow that number to keep growing. We must be really strong at this moment."
The mayor warned against holiday travel and cautioned against big Halloween parties and indoor events.
"We've got to buckle down," he said. "We have to do things right."
Trick-or-treating outdoors while wearing a mask and maintaining social distancing will still be permitted, de Blasio said, but he insisted he will close New York City schools if the seven-day average reaches 3%.
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More restrictions would also pose a problem for restaurant and bar owners, who are already struggling amid the pandemic. If they are forced to close again, many say they may not survive they year.
"Major cities in Europe are sort of sister cities, are now going back to severe restrictions, are effectively returning to pause," Senior Advisor for Public Health Dr. Jay Varma said. "And it's very important for us to take all those individual measures."
City officials say the spike could be spreading from red zone areas in Brooklyn and Queens, as well as from those not taking proper precautions.
"People really should not travel for the holidays unless it's absolutely necessary," de Blasio said. "Because unfortunately, pretty much everywhere else is doing worse at fighting the coronavirus. And if you go someplace else, the chance of bringing it back with you is high."
Any decision to restrict or expand indoor dining should these rates continue to climb will be made by the state. Right now, as a borough, Staten Island has the highest numbers in the city with 2.4%.
Governor Andrew Cuomo is also keeping a close eye on city numbers.
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