Today Mayor Bill de Blasio announced plans to change a rule which has frequently resulted in public school closures in New York City. The “two-case rule” is exactly what it sounds like. If two cases of the virus are detected in the school at the same time, that school gets shut down for 10 days. It’s a rule that the teacher’s union supports but which many parents think is unnecessary. The NY Times reports that de Blasio can unilaterally change that rule and he’s been saying for a couple of weeks now that he might do so, but prior to today he hesitated for fear of alienating the union.

Mayor Bill de Blasio will change a rule that has, for months, created a paradox in New York City’s school reopening plan: Classrooms that had been reopened to students often closed again because school buildings had to shut temporarily whenever two unrelated virus cases were detected.

The mayor announced Monday that he would alter the rule, but he did not explain how. He said the new rules will be outlined in the coming days, but did not commit to making changes this week.

The mayor isn’t saying exactly what prompted his sudden change of heart but it seems likely it had something to do with a piece ProPublica published Friday. The author is a parent with two young children who attend NYC schools. After the fifth time his kids’ elementary school was temporarily shuttered because of the two-case rule, he decided to contact epidemiologists and health experts to see what they had to say about it. The response was nearly unanimous:

So, what do scientists think of the two-case rule? “Oh my goodness. Oh my goodness. Wow. New York City schools are making a mistake,” said Dr. Elissa Schechter-Perkins, an epidemiologist and infectious diseases doctor at Boston University School of Medicine. Schechter-Perkins and her colleagues recently garnered epidemiological fame after finding that schools can reduce spacing in classrooms from 6 feet to 3 feet, a conclusion that caused the Centers for Disease Control and Prevention to change its guidelines.

What really surprised Schechter-Perkins and others were the specifics of the rule, which you can read yourself: When the city’s surveillance testing — 20% of a school’s population every week — finds two or more positive cases that are connected, just the affected classrooms are shut and close contacts are told to quarantine. But if the cases can’t be traced or connected, then the entire school building will be shut for 10 days.

Put another way: A school won’t be shut down if there is evidence of some spread, but it will be closed if there is no evidence of in-school transmission…

Other experts had similar reactions. “That’s crazy. That’s nuts,” said Dr. Benjamin Linas, another epidemiology professor at Boston University School of Medicine. “It doesn’t really make sense,” said Dr. Leana Wen, a public health professor at George Washington University who served as Baltimore’s health commissioner. “It’s not evidence driven,” said Eyal Oren, an epidemiologist at the San Diego State University School of Health.

Out of ten experts the author spoke with, he found one who agreed the two-case rule could made sense if the goal was to keep the spread of the virus near zero. However, when the same doctor was asked how he would run things if he were in charge, he replied, “Keep K through 5 in school even when there’s high community transmission.” He also pointed out the zero-tolerance police made little sense in the broader scheme of things: “Why do schools — a low-risk population, particularly with teachers vaccinated — have a zero transmission policy when the bar and gym across the street doesn’t have anything like that?”

To sum up, none of the experts thought the current policy made much sense. That story came out Friday and today the Mayor, after weeks of hemming and hawing, said he’s definitely going to change the two-case rule. I’m guessing that’s not a coincidence. The teacher’s union still won’t like his plan to change the rule, but we’re well past the point where anyone should be listening to what the teacher’s unions have to say about opening schools.