New York City announced on Thursday an end to its rule that had led to thousands of public school closures despite little evidence of COVID-19 outbreaks. Schools will now only be closed if testing shows there is viral spread within a school.

Under the old rule, schools were closed if testing found two positive results, regardless of the school’s size and even if the cases were apparently unlinked, such as those involving kids grades apart who never crossed paths. ProPublica contacted 10 epidemiologists and physicians, nearly all of whom said the policy didn’t make sense. “It’s ridiculous. Obviously ridiculous,” said Dr. Uché Blackstock, a former professor at New York University who now runs a firm focused on addressing racial inequities in health care.

Going forward, schools will only be closed if they have four or more positive tests that, critically, are traced to in-school transmission.

Research has found that schools can be safely reopened when protective measures are put in place, such as universal masking and good ventilation. Of the positive tests at schools that have been traced, most have resulted not from outbreaks within school buildings but from spread outside of them.

New York City Mayor Bill de Blasio first said back in early February that he would be revisiting the two-case rule. But changes were opposed by the city’s teachers union. Last week, the United Federation of Teachers told ProPublica it remained supportive of the policy.

But the union today expressed support for the changes. “With our increased knowledge about the spread of the virus, and as more teachers and other school staff have been vaccinated,” it said in a statement, “our medical experts are convinced that the rule can be changed and still maintain safety.”

“We worked together with the unions to figure out the right approach going forward,” de Blasio said at a press conference announcing the changes on Thursday.

The tensions in New York City echo the debates over school reopenings across the country. While an increasing number of schools have brought kids back for in-classroom teaching, many communities have moved more slowly. The majority of children in California still do not have access to in-class instruction.

Instead of closing when two cases are found in different classrooms, New York City schools will conduct further testing. But individual classrooms will still be closed when testing finds at least one positive case.

Dr. Elissa Schechter-Perkins, an epidemiologist and infectious disease doctor at Boston University School of Medicine, said she’s concerned the new policy will still result in children unnecessarily losing access to in-person schooling.

“Shutting down a class after only the introduction of the virus into the classroom is not necessary,” said Schechter-Perkins, whose research recently prompted the Centers for Disease Control and Prevention to revise its guidance on spacing in classrooms. “We have ample evidence that the rate of in-classroom secondary transmission from that index case is exceedingly low.”