The Centers for Disease Control and Prevention on Friday urged that K-12 schools be reopened as soon as possible, and it offered a step-by-step plan to get students back in classrooms and to resolve a debate dividing communities across the nation.
The guidelines highlight growing evidence that schools can open safely if they use measures designed to slow the coronavirus’s spread. The agency said that even in communities with high transmission rates, elementary-school students may receive at least some in-person instruction safely.
Middle and high school students, the agency said, may attend in-person classes safely when the virus is less prevalent, but may need to switch to hybrid or remote learning in communities experiencing intense outbreaks.
“C.D.C.’s operational strategy is grounded in science and the best available evidence,” Dr. Rochelle Walensky, director of the C.D.C., said on Friday in a call with reporters.
The guidelines arrive amid an intensifying debate. Even as parents in some districts grow frustrated with shuttered schools, some teachers and their unions refuse to return to classrooms they regard as unsafe.
Public school enrollment has declined in many districts. Education and civil rights leaders are worried about the harm to children who have not been in classrooms for nearly a year.
The recommendations tread a middle ground between those eager to see a resumption of in-person learning and those fearful that schools reopenings will spread the virus.
In advice that may disappoint some teachers, the document states that vaccinating educators should be priority, but not a prerequisite for reopening schools.
Still, both national unions thanked the C.D.C. for the clearer guidance.
“For the first time since the start of this pandemic, we have a rigorous road map, based on science, that our members can use to fight for a safe reopening,” said Randi Weingarten, president of the American Federation of Teachers and an ally of President Biden.
But Ms. Weingarten and Becky Pringle, president of the National Education Association, argued that schools might find the C.D.C.’s mitigation strategies difficult to enact without additional federal funding.
The agency’s guidance repeats the idea that schools should be the last to close and the first to reopen in any community. But the C.D.C. has no power to force communities to take steps to decrease high transmission rates — such as closing nonessential businesses — in order to reopen schools.
By the agency’s new criteria, schools in more than 90 percent of U.S. counties could not return to in-person classrooms full-time, Dr. Walensky noted. Nonetheless, the majority of districts are offering at least some in-person learning, and about half of the nation’s students are learning in classrooms.
But there are stark disparities in who has access to in-person instruction, with urban districts serving mostly poor, nonwhite children more likely to have closed schools than suburban and rural ones.
Researchers are concerned not just about the academic consequences of being out of school for such a prolonged period. While data are still very limited, many doctors and mental health experts report seeing unusually high numbers of children and adolescents who are depressed, anxious or experiencing other mental health issues.
The agency’s approach struck the right balance between the risks and the benefits of in-person instruction, said Jennifer Nuzzo, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health.
“We have accumulated a tremendous amount of harms from not having schools open,” Dr. Nuzzo said. “This document is important in trying to couch the risks in relation to those harms, and try to paint a path forward.”
The C.D.C. offered advice to school administrators tailored to four levels of viral transmission in the surrounding communities.
The agency said that elementary schools could remain open regardless of virus levels in the surrounding community, pointing to evidence that young students are least likely to be infected or to spread the pathogen.
Only in communities with the highest transmission levels should elementary schools switch to a hybrid model, with some remote instruction and some in-person instruction, the agency said. In any scenario, elementary schools should remain at least partly open. Middle schools and high schools should close completely and switch to virtual learning when transmission levels are highest, the agency said.
The guidelines also prioritized in-person instruction over extracurricular activities like sports and school events. In an outbreak, these activities should be curtailed before classrooms are closed, officials said.
Some experts raised concerns about the strategy. Many schools in communities where viral transmission is high have been open for fully in-person instruction, without experiencing outbreaks of the virus.
Absent from the agency’s guidance were detailed recommendations on improving ventilation in schools, an important safeguard.
In one short paragraph, the C.D.C. suggested that schools open windows and doors to increase circulation but said they should not be opened “if doing so poses a safety risk or a health risk.”
“C.D.C. gives lip service to ventilation in its report, and you have to search to find it,” said Joseph Allen, an expert on building safety at the Harvard T.H. Chan School of Public Health in Boston. “It’s not as prominent as it should be.”
Other preventive measures the C.D.C. recommended for schools are those it has previously endorsed. Universal wearing of masks and physical distancing are the most effective, but the agency also endorsed hand-washing and hygiene, cleaning and contact tracing.
The agency advised that schools refer all symptomatic students, teachers, staff and their close contacts for diagnostic testing, and that schools consider routine weekly testing of students and staff, except in communities where transmission is low. The expense and logistics of widespread screening would be a heavy burden for school districts, some experts noted.
The C.D.C. said in communities with higher levels of transmission, schools should make sure individuals maintain at least six feet of physical distance. But in communities with lower levels of transmission, the agency said that students and staff should be physically distanced only “to the greatest extent possible.”
“We’re worried that people will not be able to get back to full in-person learning if we mandate six feet of physical distance,” Dr. Walensky acknowledged.
“A lot of communities have pursued hybrid approaches or, in some cases, just not opened, because they haven’t been able to figure out that spacing issue,” said Dr. Nuzzo of the Johns Hopkins Bloomberg School of Public Health. “The whole attempt to bring kids back to school doesn’t have to break down over that.”
But Ms. Pringle of the National Education Association, the country’s largest teachers’ union, said there should be no wiggle room on physical distancing or other mitigation strategies.
“We need detailed guidance from the C.D.C. that doesn’t leave room for political games,” she said. “This is an airborne disease. Masks must be mandated, social distancing must be in place and proper ventilation is a must.”
As it had previously, the C.D.C. recommended using two measures to determine the risk of transmission in the community: the total number of new cases per 100,000 people, and the percentage of positive test results over the previous seven days.
Dr. Helen Jenkins, an infectious disease expert at Boston University, said the percentage of positive tests can vary with how much testing a community is doing. And the highest levels of community spread defined by the agency are too conservative; schools would be safe even if there were more cases in the community, she and other experts said.
Mr. Biden has pledged to open a majority of K-8 schools within the first 100 days of his administration. But on Wednesday, the White House press secretary, Jen Psaki, said that the president had been referring to in-person teaching “at least one day a week.”
According to the agency’s new guidelines, many schools now operating virtually should consider at least some in-person learning.
If the new recommendations had been in place last fall, for example, San Francisco could have opened all of its schools for fully in-person instruction in mid-September. Today, according to the guidelines, San Francisco could open elementary schools in a hybrid mode, and the city is close to being able to open middle schools and high schools in a hybrid mode.
Instead, the city’s schools have been shuttered since the pandemic began, and the district has agreed to far more restrictive reopening standards with its union. Officials have set no date for bringing young children back to school, and have said they do not expect most middle- and high-school students to return in person this year.
The new guidance recommended that states immunize teachers in early phases of the rollout but said access to vaccines should “nevertheless not be considered a condition for reopening schools for in-person instruction.”
Vaccinating teachers is very effective at reducing cases among both teachers and students in a model of transmission in high schools, said Carl Bergstrom, an infectious diseases expert at the University of Washington in Seattle. “It should be an absolute priority,” he said.
Still, he added, “I can certainly see why they chose not to make it a prerequisite, because it may not be something that can be done in time to have schools open.”
Some teachers’ unions have also asked for stringent protections regarding air quality inside school buildings, an issue not fully addressed by the C.D.C.
In Boston, for example, air quality was a major point of contention in reopening negotiations between the school district and teachers’ union. Their agreement called for air purifiers in classrooms and a system for testing and reporting air quality data.
Ms. Pringle, the union president, said her members continued to be concerned about aging schools that did not have modern ventilation systems. Those buildings were more likely to be in lower-income and nonwhite communities hit hardest by the pandemic.
On Friday, Dr. Walensky said while the new guidelines should enable schools to stay open through most local conditions, if transmission skyrockets — perhaps because of the contagious new variants beginning to circulate in the country — “we may need to revisit this again.”