The Centers for Disease Control and Prevention revised its guidelines for schools Friday, saying 3 feet of distance between students is sufficient for all elementary and most middle and high schools, a change that lays the groundwork for many districts to reopen full-time for in-person classes. 

The announcement came as the CDC published new research that found limited coronavirus transmission in schools that require masks but not always 6 feet of distance, which had been the standard. That was true even in areas with high community spread of the virus.

Teachers unions opposed the change, and local unions may resist any effort to bring large numbers of students into the building at one time. Many large districts have just recently begun to reopen for part-time, in-person school, and often after tense negotiations with teachers.

Nonetheless, the new guidelines represent a significant reversal from CDC guidance issued last month that schools maintain 6 feet of distance between people. To achieve that, CDC said schools in most of the country should hold off on fully reopening.

That put the CDC at odds with President Biden, who has called on schools to fully reopen.

The Februay recommendation also came under fire from many experts as overly cautious, particularly as more evidence emerged that schools were safely operating with people closer to one another. Nonetheless, with the guidance in place, many districts adopted hybrid systems, where students are in school buildings part of the time and learning from home the rest.

CDC Director Rochelle Walensky said Friday that new evidence prompted the change.

"CDC is committed to leading with science and updating our guidance as new evidence emerges," she said in a statement.

Until recently, the debate over distancing in schools has been complicated by a lack of research directly addressing the risks of various distances between people. Most researchers agree that the research behind 6-foot parameter is outdated, but they also agree that more distance is better.

A growing number of scientists have called for lower distances in schools, saying the risk must be weighed against growing examples of safe reopening and mounting evidence of mental health and academic harms to students who have been learning remotely for more than a year.

"Look, 100 feet is safer than six feet, which is safer than three feet," former CDC director Tom Frieden said during a Washington Post Live interview this week. "Is three feet okay for most schools? Absolutely, if they mask, if they repeatedly identify cases and isolate and quarantine."

That case was bolstered last week with a study, published in Clinical Infectious Diseases, that found similar rates of spread in Massachusetts school districts that used a 3-feet minimum and those that used 6.

Friday's change comes after some state and local officials have dropped the 6-foot recommendation on their own. This week, for instance, Loudoun County Public Schools in Virginia said it would open four days a week in April, a change made possible by reducing the required distance from six to three feet.

Others said the new rules would speed a return to more normal school.

"Like everyone else, we're waiting for the CDC to change its social distancing guidelines so we can go to three feet," said Roberto Padilla, superintendent of the Newburgh, N.Y., city schools. He said he hopes to ramp up from a hybrid system to full-time this spring.

In Philadelphia, Superintendent William Hite also hopes the recommendations will help him ramp up in-person teaching, but the district has a long way to go. Currently, an agreement with the teachers union allows only for children in pre-K through second grade in buildings and only part-time. "We're going to quickly pivot to reanalyzing distancing for the classrooms and schools so that we can open those rooms and schools to more children," Hite said Friday.

But the change is opposed by the country's two large teachers' unions, and it's far from clear that teachers will go along. Ahead of the announcement, the unions argued that there is scant research about the impact of closer contact in urban schools, where buildings are older and classrooms more crowded.

"We are concerned that the CDC has changed one of the basic rules for how to ensure school safety without demonstrating certainty that the change is justified by the science," said Becky Pringle, president of the National Education Association.

The CDC said Friday that most of the school-based infections have been between adults, or between students and staff. Therefore, it recommended 6 feet of distance between students and their teachers and among teachers and staff. But the CDC said the rules can be relaxed for student-to-student interactions.

For elementary schools, it said, 3 feet of distance among students is sufficient no matter what the infection rates are in the surrounding community. Young children are much less likely to have severe cases of COVID-19, and some research suggests they spread the coronavirus less efficiently than adolescents and teens.

The recommendations are more complex for middle and high schools and depend on which of four levels of community transmission are present in the surrounding area. At the three lowest levels, the CDC says 3 feet of distance is sufficient for all schools. But at the highest tier, the agency recommends 6 feet - meaning schools would probably have to rotate students in a hybrid system. As of Sunday, CDC data showed that 41 percent of U.S. counties were in that highest tier, defined as a seven-day average of 100 or more daily cases per 100,000 people.

The agency says that even then, secondary schools can drop the standard to 3 feet, but only if they are able to keep students in cohorts, which limit interactions among a small group. That is difficult to do in middle and high school, where students typically mix up into different groups depending on the course.

This nuance is helpful, said Joseph Allen, an associate professor at the Harvard T.H. Chan School of Public Health. He said the guidance and the studies published now make clear that in-school infections can be avoided with proper measures in place.

"Even with high community spread, you can control within-building transmission," he said.

The guidance also made a range of other changes that Allen praised, including a greater emphasis on ventilation and the removal of a recommendation to use Plexiglas and other physical barriers, the use of which he said is not supported by evidence.

The CDC published three new studies Friday that appeared to augment the evidence that schools can operate safely, even where community spread is high, as long as masking and other measures are used.

One, carried out in an urban district of Salt Lake County, Utah, examined transmission in 20 elementary schools over a six-week period in December and January. Mask use among more than 10,000 students and 1,200 staff was high and students were grouped in classroom cohorts. The average distance between the children was 3 feet, the study said.

Fifty-one index, or primary, coronavirus cases were identified, and unlike in previous studies of school-based transmission, the researchers tested all 735 people determined to be close school contacts of the initial 51 - an important step in finding asymptomatic cases that can still transmit the virus. Researchers identified just five cases of in-school transmission, which, they wrote, "strengthens the evidence for low elementary transmission."

The results, the authors concluded, suggest that even when students are spaced less than six feet apart, "schools in high-incidence communities can still limit in-school transmission by mask use and ensuring other important strategies to reduce transmission risk are implemented."

A second study looked at in-school transmission over two December weeks in 55 K-12 schools in Saint Louis County and Springfield, Mo. Nearly all the schools implemented multiple mitigation measures - masking, ventilation upgrades and handwashing stations. But spacing between students varied, with many schools using a minimum of three feet. Although community spread was high during the period - more than 700 daily cases per 100,000 people - researchers identified no school outbreaks and just two cases of in-school spread.