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For months, school districts throughout the country have struggled with whether and how to reopen buildings that, in some cases, have been shuttered for nearly a year. With bullish talk and promised support, President Biden raised expectations that reopenings would accelerate this spring.

The stakes are enormous. Going back is frightening for many teachers and parents, especially with coronavirus rates remaining at high levels and new variants of the virus emerging. And yet the negative consequences of all-remote learning are significant, too. Children are forfeiting academic progress and struggling emotionally. Some parents are unable to work while their children are at home.

But sorting out the science of the matter has been complicated for administrators and parents. The Centers for Disease Control and Prevention weighed in last week with a detailed set of guidelines that answered many questions but raised others.

What do we know about coronavirus transmission in schools?

In a review of the science, the CDC found in-person schooling has not been associated with substantial transmission in the wider community. Multiple studies found transmission rates inside schools are similar to, or lower than, levels in the community when mitigation steps are in place.

A study of 11 North Carolina school districts holding in-person learning last fall found minimal transmission even when community transmission was high. These schools implemented and adhered to strict mitigation strategies, including universal mask use and physical distancing inside buildings, the CDC noted. Similar results were found in studies in Italy, Switzerland, Chicago and elsewhere.

When school-based transmission has been reported, it has been more common between adults than from or among students. And while children can get covid-19, the disease caused by the novel coronavirus, they are less likely to have symptoms, and growing evidence shows that those younger than 10 transmit it less efficiently than older children.

What precautions are needed to safely return to school?

The CDC has laid out protocols for a safe return to schools. Most important is mandatory and proper use of masks for all people in the building. Other key policies include proper handwashing, with adequate supplies to do so; cleaning and maintaining facilities; and contact tracing to ensure that people in close contact with those who are infected quarantine at home.

The CDC also emphasizes physical distancing in schools, saying that six feet should be established between people “to the greatest extent possible.” In practical terms, the agency recommends that areas of high community transmission adopt entirely remote school or hybrid programs, in which students learn in person sometimes and at home other times, to ensure proper distancing. But in areas with low levels of transmission, the CDC says less space between people may be okay.

Daily coronavirus case counts have been falling significantly, with the rates now less than half what they were. Nonetheless, the CDC adopted a strict measure of community spread that puts the vast majority of the country into the “high transmission” zone, the worst of four color-coded zones.

Is six feet of distancing really necessary?

The CDC’s new guidance recommends placing desks six feet apart “when feasible.” But other public health bodies suggest less strict standards. The World Health Organization recommends people in schools stay at least one meter — about three feet — apart. And some schools, such as those in Indiana, have already adopted a three-foot guideline.

The challenge with strict six-foot guidelines is that they require smaller classes and make full-time school all but impossible. For that reason, the American Academy of Pediatrics says greater distance “may outweigh the potential benefits” if it reduces classroom time. The medical group recommends student desks be at least three feet apart, and that adults, who are more susceptible to the coronavirus than kids, stay six feet from others.

Some researchers say the six-foot standard is based on dated science. While large respiratory droplets typically fall to the ground within six feet, scientists now know that the aerosols that play a role in coronavirus transmission can travel farther. Even so, these researchers say, transmission can be dramatically reduced when layered with other mitigation measures adopted since the start of the pandemic, such as mask-wearing.

What about ventilation in classrooms?

The CDC’s new guidance includes just one paragraph on ventilation, which it says should be improved “to the extent possible” by opening windows and doors. The agency previously offered more detailed recommendations, such as portable HEPA filters and upgrades to heating and cooling systems.

But many experts say good ventilation is key to controlling a virus known to be airborne. The CDC guidance underplays the importance of ventilation while overemphasizing cleaning in schools, even though surface transmission is no longer thought to significantly contribute to spread, two researchers argued in a recent Washington Post column. “Shared air is the problem, not shared surfaces,” wrote Joseph G. Allen of Harvard University and Helen Jenkins of Boston University.

Is it safe to have all children back in school or should schools adopt hybrid systems with students learning part time from home?

Some school districts say it’s too risky to reopen at all, or have struggled to create and adopt workable reopening plans. But many school districts are using hybrid systems as a way to ensure six feet of distance between students and staff.

Hybrid programs are popular in many communities as a compromise between fully remote and fully in-person learning, allowing for some face-to-face instruction. But they are complicated because teachers sometimes must instruct students in the building and at home at the same time. And some say they introduce risk because students may be mixing with others during their at-home time and then bringing possible infections into the schools.

In general, in-person learning is considered safer for elementary school, partly because students are more easily kept in cohorts, where they come in contact with a limited set of people. In addition, young children may transmit the virus less efficiently, reducing the risk.

Should teachers be vaccinated before going back to school, and what’s the timeline for that?

Some teachers unions have called for educators to be vaccinated before returning to classrooms. A CDC advisory committee recommends that states put them into the second-highest priority level, with other essential workers. But where teachers fall in line varies widely state to state, and experts say it could be months before all teachers get the shots.

The vaccine is available to educators in at least 28 states, along with D.C. and Puerto Rico, according to an Education Week tally. Demand for the vaccine continues to outpace supply, and many teachers may not be inoculated until the spring, nearing the end of the school year.

The CDC said vaccination is a good strategy to enable schools to reopen but not a prerequisite. Rather, the agency said schools can operate safely through mitigation measures, including masking, maintaining distance and contact tracing.

“I think we have to keep remembering that there are risks to kids not being back in school, and they’re not small,” said Sara M. Bode, a pediatrician at Nationwide Children’s Hospital in Ohio. “They’re great.”

When will children be vaccinated?

Anthony S. Fauci, the federal government’s top infectious-disease expert, told ProPublica last week that children as young as first-graders could be vaccinated by September. But that may be optimistic.

Several vaccines have been approved for adults worldwide, and manufacturers are testing them in successively younger groups. Pfizer-BioNTech, the maker of one of two vaccines approved for adults in the United States, is conducting a clinical trial for children ages 12 to 15. Moderna, which makes the second U.S.-authorized vaccine, is still enrolling a trial for youngsters ages 12 to 17. Both companies have said they expect results in the summer, which could mean adolescents and teens will be eligible for vaccination before the fall.

Only after those results come in will trials for younger children start, so the timeline for them remains unclear. Moderna’s CEO has said he does not expect data on the safety and effectiveness of its vaccine in children younger than 11 to be available until 2022.

In the United Kingdom, Oxford University this month began testing the vaccine it co-developed with drugmaker Astra-Zeneca on children ages 6 to 17 —- the youngest age group yet.

What impact should the emergence of variants have on sending students back?

The emergence of more transmissible variants of the coronavirus has raised fresh concerns over returning to school. In Europe, some countries that prioritized keeping classrooms open reversed course, with officials citing the threat of the variant first identified in Britain.

The CDC’s guidance on reopening schools was released after the variants were identified in the United States, and experts say following the agency’s layered mitigation strategy should keep the risk of transmission relatively low. The arrival of the variants means teachers and students should double down on the measures the CDC released, including maintaining distance and wearing masks. They should also ensure that face coverings are closefitting and worn at all times, except when a person is eating or drinking.

As more information becomes available about the variants, that advice could change, Bode said. But with what’s known at this stage, she said, “I think the important thing is getting kids back to school.”

How many students have access to in-person classes today?

There is no federal data, and it is not totally clear, though the trend is toward more in-person learning, with several large districts beginning to open buildings.

The private firm Burbio surveys 1,200 school districts a week to make national estimates. It reports about 41 percent of U.S. students are attending traditional programs, with school in person every day, with about 26 percent in a hybrid setting. It estimates that about 34 percent attend all-virtual schools.

Another company, MCH Strategic Data, reported a few weeks ago that about half of districts were offering hybrid programs and about 1 in 4 were fully online.

It’s not known how many students are learning in classrooms because most districts offer an all-remote option, and there is no tally of how many students take that option. The U.S. Department of Education recently announced that it would conduct a survey of its own to learn more about the status of school systems.