Mayor Bill de Blasio on Tuesday said that New York City's daily testing positivity rate for COVID-19 had reached 3.25%, a jump of more than a percentage point over the previous day.

In a cruel irony, the sudden rise in the city's positivity rate comes on the first day of in-person instruction for about 300,000 children attending K-5 and K-8 schools across the city. Prior to Monday, the number of people testing positive has hovered around 1 percent for around two months.

De Blasio attributed the uptick to 9 ZIP codes in Brooklyn and Queens, which are home to significant numbers of Orthodox residents. While health officials have targeted outreach to those communities, the mayor has refrained from citing a specific source for the infections. He said that health officials would work to expand testing and strengthen enforcement, but that schools would in all neighborhoods continue to reopen.

But in a sign of a possible pushback from educators, Michael Mulgrew, the head of the teachers union, on Tuesday expressed concern about reopening schools in Brooklyn neighborhoods with covid clusters.

“We are very concerned with Brooklyn and those (zip) codes in Brooklyn. If we don’t see those numbers start coming down by the end of the week we are going to get much more aggressive with City Hall,” he said.

He later added: “Everything goes on the table. I know that makes people uncomfortable when we say those things but it’s a fact. We cannot, cannot allow politics to get in the way of our safety concerns at this point in time.”

At the end of July, the mayor had said that schools would not reopen if the positivity rate on a seven-day rolling average rose to 3%.

That seven day average now stands at 1.38%.

"I said very clearly when we announced that it would be the seven day rolling average," he said.

But the standard was confusing because during his press briefings, the mayor has read daily indicators. In a switch, he said he would now report the positivity rate as a seven-day rolling average. This is the second time the administration has changed the way it reports or frames its coronavirus daily updates. In April, de Blasio introduced "indicator thresholds" that would determine the city's reopening after initially saying that the city wanted all three indicators—hospitalizations, ICU cases, and the positivity rate—to move together.

On Tuesday, the mayor stressed that the city was home to 146 ZIP codes, the majority of which were not showing spikes in infections. He urged city residents to get tested. "We need a clear picture of what’s happening in the city," he said.

The city will begin fining those who refuse to wear a mask when offered one and order any private schools or child care centers to close that fail to follow public health guidelines, de Blasio also announced.

Governor Andrew Cuomo, who also held a press conference on Tuesday morning, said that there were now 20 ZIP codes in the state which together have an average positivity rate of 5%, or five times the positivity rate of the statewide average.

"These are embers that are starting to catch fire in dry grass," Cuomo warned. "Send all the fire fighting equipment and personnel to those embers and stamp out the embers right away."

The governor said that he planned to meet with members of Orthodox community, emphasizing that public health rules apply to everyone. The governor, a self-identified Catholic, reminded the public that he canceled the St. Patrick's Day Parade even though the event is considered "near and dear" to Catholics.

"I love and respect the Jewish community," he later added. "These laws apply to everyone and these laws will be enforced against everyone."

Cuomo stopped short of announcing any rollback on the reopening, but implored local governments, specifically in NYC, Rockland County and Orange County, to enforce existing protocols on mask compliance, religious gatherings, and bars and restaurants.

"The local government failed to do its compliance job," Cuomo said. "That's how the cluster happened. Let's be honest. There were pictures of people violating compliance and not wearing masks."

David Cruz contributed reporting.