(CNN) The US Centers for Disease Control and Prevention on Friday released guidelines for reopening schools that focus on five key Covid-19 mitigation strategies: the universal and correct wearing of masks; physical distancing; washing hands; cleaning facilities and improving ventilation; and contact tracing, isolation and quarantine.
Vaccines and testing are not among the "key" strategies the agency lays out, calling them "additional layers" of Covid-19 prevention.
But the new recommendations come amid a national debate about when and how to reopen schools, even as fear of spreading the coronavirus continues and a push to prioritize teachers for vaccinations grows.
"I want to be clear, with this operational strategy, CDC is not mandating that schools reopen. These recommendations simply provide schools a long-needed roadmap for how to do so safely under different levels of disease in the community," CDC Director Dr. Rochelle Walensky said in a news briefing on Friday.
"We also know that some schools are already providing in-person instruction and we want them to be able to continue to do this, but we know that some are not following the recommended mitigation strategies we know to work," Walensky said. "For these schools, we are not mandating that they close; rather, we are providing these recommendations and highlighting the science behind them to help schools create an environment that is safe for schools, students, teachers and staff."
Walensky added that while each strategy is important, CDC recommends "prioritizing the first two" -- wearing masks and physical distancing.
"These two strategies are incredibly important in areas that have high community spread of Covid-19, which right now is the vast majority of communities in the United States," Walensky said. The recommendations also emphasize keeping students in cohorts or "pods" to limit their contact with others and encourage physical distancing.
The new recommendations also note that the risk of Covid-19 spread in a school can be associated with how much spread is in the surrounding community.
"I want to underscore that the safest way to open schools is to ensure that there is as little disease as possible in the community. We know that the introduction and subsequent transmission of Covid-19 in schools is connected to and facilitated by transmission of COVID-19 in the community," Walensky said on Friday.
The CDC recommendations include a color-coded chart to describe levels of transmission from blue for low transmission, to yellow for moderate, to orange for substantial and then to red for high transmission.
President Joe Biden said in a statement on Friday that the new CDC guidelines provide "the best available scientific evidence" on how to reopen schools safely, which remains one of his goals.
"We have sacrificed so much in the last year. But science tells us that if we support our children, educators, and communities with the resources they need, we can get kids back to school safely in more parts of the country sooner," Biden said in part.
"When my Secretary of Education is confirmed, I will task him with working alongside school administrators, educators, and parents to safely accelerate the process of school reopenings. As many states continue to follow the CDC's recommendation to prioritize teachers for vaccination, I urge all states to follow suit."
Biden has said he will work to reopen most K-12 schools within his first 100 days in office but has stressed he will rely on health and medical experts to dictate the national guidance in order to reopen safely.
"It is a challenge to guarantee, or to say that our schools could open in 100 days -- because of the deep investments that need to be made around what it takes to get schools ready and prepared to reopen," Annette Anderson, a professor in the Johns Hopkins University School of Education and serves as deputy director for the new Center for Safe and Healthy Schools, told CNN on Friday.
"What I'm struck by is this notion that, even with the layered mitigation that they are suggesting is really an important strategy, that the CDC is not mandating that schools reopen -- that's a critical piece of this conversation because I think there was this expectation that after the 100 days, there was going to be some kind of opening that would mean all students would go back immediately," Anderson said.
"We now know that this is going to be a much more nuanced process, and it will take time," she added. "I think we can be hopeful that if schools adopt these strategies and as they continue to gradually reopen, that by fall, we will see more of our students in the classrooms for face-to-face learning."
In the new guidelines, the CDC does not list vaccination as a "key" strategy for opening schools, focusing instead on measures such as masks and physical distancing. Vaccinations for staff and teachers are "an additional layer of protection," Walensky said.
She added that the CDC's Advisory Committee on Immunization Practices recommends that frontline essential workers -- a group that includes educators -- be prioritized for Covid-19 vaccination.
"As such, we strongly encourage states to prioritize teachers and other school staff to get vaccinated," Walensky said. "If we want our children to receive in-person instruction, we must ensure that teachers and school staff are healthy and protected from getting Covid-19 in places outside of schools where they might be at higher risk."
Anderson said that vaccines are important to help rebuild trust with families, especially those who are skeptical that schools are ready to reopen.
"That's the work of our federal government right now, to help to convince families that they have provided the support for reopening, and that they have a tiered strategy that is going to help the schools to systematically follow a protocol that can be trusted," Anderson said. "That was missing a year ago."
Randi Weingarten, president of the American Federation of Teachers (AFT), called the CDC's new recommendations an "informed" and "rigorous" roadmap for the federation's members to use. Weingarten added that AFT remains supportive of widespread testing, which the guidance is "instructive" for, and noted that the guidance reinforces prioritizing vaccines for teachers and school staff.
"Of course, this set of safeguards should have been done 10 months ago—and the AFT released its plan recommending a suite of similar reopening measures in April. Instead, the previous administration meddled with the facts and stoked mass chaos and confusion. Now we have the chance for a rapid reset," Weingarten said in a written statement on Friday.
Several CDC officials have complained that the Trump White House interfered with guidance, even reviewing scientific documents. This new CDC guidance is based on science and free of political interference, Walensky said.
"I can assure you that this is free from political meddling," she told reporters on Friday.
"We have presented pieces of this scientific guidance to the White House so that they would know what we are planning, but they have not reviewed my remarks," Walensky added.
Walensky said Friday's guidelines were based on research, including data from the European Union, where more schools have reopened to in-person learning than in the United States, as well as from the few schools that have reopened in the US.
"The guidance is instructive for this moment in time, but this disease is not static," Walensky said in part. "The stage is now set for Congress and the Education Department to make this guidance real -- and that means securing the funding to get this done in the nation's school districts and meet the social, emotional and academic needs of kids."
Walensky on Friday said that the CDC grounded its new recommendations in science and "the best available evidence." The agency also released a scientific brief to accompany its new strategies for schools. The brief includes references to CDC data and separate studies published in journals such as Archives of Disease in Childhood and Lancet Infectious Diseases, as well as some pre-print research.
A CDC science brief posted on Friday notes, "Based on the data available, in-person learning in schools has not been associated with substantial community transmission."
Rather, "when community rates of COVID-19 are high, there is an increased likelihood that SARS-CoV-2 will be introduced to, and potentially transmitted within, a school setting. Evidence to date suggests that when schools implement mitigation strategies with fidelity, transmission within schools can be limited," according to the scientific brief. SARS-CoV-2 is the name of the virus that causes Covid-19.
"We have engaged with many education and public health partners to hear first-hand from parents and teachers directly about their experiences and concerns and have conducted an in-depth review of the available science and evidence to guide our recommendations," Walensky said.
Previous studies suggest that some schools have managed to implement full or part-time in-person learning, without the kind of Covid-19 spread seen in crowded offices or long-term care facilities. Transmission has occurred, but CDC researchers say there is little evidence that it has contributed meaningfully to increased community spread.
In one CDC study, mitigation measures, including social distancing, contact tracing and wearing masks -- provided to students through a grant from a private foundation -- helped 17 rural schools in Wisconsin achieve transmission rates that were 37% lower than those of the community at large. Of 191 Covid-19 cases, just 3.7% were contracted in school.
Those mitigation measures can make a big difference. Another CDC study detailed how two Florida high school wrestling matches -- a high-contact sport that does not allow for masking or social distancing -- became superspreader events that led to the loss of an estimated 1,700 in-person school days.
Overall, more research on the public health impacts of holding in-person learning during the pandemic remains needed, said Anderson of Johns Hopkins University.
"The challenge is that we've had so many stops and starts around what research to follow. ... We've heard research that suggests that schools are safe and kids are not super spreaders, and then you hear that because of the Covid-19 variants, maybe kids are super spreaders," Anderson said.
"The research will continue to come out and over time as we start to see more consistency in all of the research and the studies that are done, it will build up the confidence of families," she said. "This is not a one and done. I think it's the beginning of a great rebound and response, but we have work to do and so we should focus on what we need to do between now and the reopening of schools in the fall so that we can have more consistency in strategy and planning, and measuring the outcomes."