In July, the Centers for Disease Control and Prevention released the latest version of covid-19 prevention guidance for K-12 schools. Parents, scientists and educators who have been advocating for a full reopening of schools breathed a sigh of relief: The updated guidance is undoubtedly a major leap forward in the efforts to return children to classrooms.

Besides unequivocally prioritizing full-time, in-person school — even if classrooms can’t accommodate social distance and even if case counts rise — the new guidance contains several other evidence-based changes. For example, children sitting in classrooms with masks at least three feet away from another student who tests positive for the coronavirus are no longer considered close contacts, and won’t be subject to quarantine. This seemingly small update to the definition of “exposure” will have a large and positive impact. Recently, a stricter definition of exposure combined with new asymptomatic screening tests in New York City led to immediate and unnecessary closures in its Summer Rise program; the CDC’s new rules would prevent that. Other improvements include a de-emphasis on surface cleaning and specific advice on opening windows and using fans to improve ventilation. Following the science means adapting as we learn more.

Where the guidance falls short is in providing a threshold for, and nuanced discussion about, removing masks for unvaccinated students and staff. Instead, the CDC left this important decision to local public health officials and school leadership. But districts around the country need guidance on how and when to pivot between mandatory and optional mask-wearing. National and state health and education officials typically set minimum standards for prevention measures. Over the past school year, however, we’ve seen the elected leadership of many individual school districts, pressured by stakeholders including parents and staff, make rules that went far beyond those standards — ostensibly in the name of safety. Many schools that could have opened remained closed.

And it can be extremely hard for districts to change course after the school year starts. In Massachusetts, many school districts mandated six feet rather than three feet of distancing between students — far stricter than the rules issued by the state education department. From the perspective of local school leaders, this mandate signaled that they were taking pandemic safety seriously. Yet evidence to support the distancing policy was always weak. From a practical standpoint, the policy was a disaster — six feet of distance meant most schools could not accommodate all of their students full time. And once that standard was incorporated into teachers union contracts for the entire academic year, reversing the decision and fully opening schools required a literal act of emergency powers.

We’re already seeing signs that this pattern will repeat next year. On July 8, the superintendent of schools in Decatur, Ga., announced that masks would be mandatory for all students in schools next year. School districts in California and New York have adopted similar policies. This insistence on universal mask-wearing is not an evidence-based approach, nor is it recommended by the CDC, either in schools or out.

Vermont, by contrast, lifted indoor mask-wearing mandates for students before the end of the last school year in response to high vaccination rates, and levels of cases and hospitalizations drastically falling from their winter peak. This might seem surprising, given that Vermont once had some of the most restrictive public health rules in the country. But as a result, it had some of the best covid outcomes — and state leaders acted accordingly. The state’s health commissioner, Mark Levine, explained that as long as the state’s infection rate remains low, he felt confident that students, including those younger than 12 who cannot yet be vaccinated, were in a very safe environment. Subsequently, the state department of health announced that schools may not mandate masks.

While some in Vermont’s school communities are rejoicing at the restoration of human connection unencumbered by face coverings, others are understandably worried. Different families have different risk-tolerance levels, and we can acknowledge and respect these differences. Many adults and their children will choose to continue using masks, even in areas with low prevalence of disease and high rates of vaccinations; others will not. But mandating policies that aren’t backed by science ultimately undermines trust, and if we have learned anything since the start of the pandemic, it’s that the erosion of trust in science and government can have dire consequences.

The CDC document states that unvaccinated individuals must continue to mask in schools, but until when? Given the low risk of severe covid infections in elementary school-age children, there will be a very high bar for approving any vaccine; its benefits would be slim compared to the benefit to adults, so the Food and Drug Administration will take any risks involved even more seriously. If the vaccine isn’t speedily approved for use in young children, that could mean elementary schoolchildren would be required to wear masks for years, a requirement divorced from any assessment of what danger they really face.

We know that the risk to any person of suffering harm from covid-19 is a function of four main factors: One, the vulnerability of that individual to a severe case of the disease, which typically comes down to age and whether they have co-morbidities. Two, the person’s likelihood of coming into close contact with another person infected with the virus, as measured by community prevalence of covid-19 infection. Three, the setting in which that close contact occurs (outdoors or indoors; with more ventilation or less). And four, the immune status of the individual — meaning vaccinated, previously infected or neither. We know children are extremely low risk for severe disease from covid-19 — a fact we should be celebrating every day, though there are exceptions.

But now there’s another piece of good news: Many places in the country with high vaccine uptake are enjoying rates of infection so low that the chances of even one person with covid-19 entering a school building are slim. Districts experiencing low infection rates during the next school year should not operate under the same restrictions as they would during times of high transmission, or with the same rules as parts of the country with surging cases.

We can’t rely on the individual discretion of school districts to produce effective, workable rules for school covid mitigation measures, nor can we rely on broad, one-size-fits-all mandates. This week, the American Academy of Pediatrics recommended that masks be worn in schools by all students and staff, regardless of vaccination status. Such blanket statements from national professional organizations have, thus far, not acknowledged the very different situations faced by various jurisdictions across the country. At the same time, local school boards lack the expertise to apply epidemiologic principles, and may be unduly influenced by political pressure. This is why it’s so important that we have metrics set at the federal level, so that local leaders know when they can safely lift various in-school measures.

Mask-wearing for students and staff in indoor classrooms should be optional, not required, as long as covid-19 hospitalizations (a more reliable measure of disease in a region than cases) remain low in a given district. Benchmarking infection control strategies to community prevalence is the approach taken by the World Health Organization, which advises strongly against masks for children 5 and younger and recommends that the decision to have 6- to 11-year-olds use masks be based on local transmission rates. The presence of highly transmissible variants and the stark differences in vaccination uptake across the country means a one-size-fits-all national or even regional approach to mask-wearing wouldn’t work.

Last year, decisions about when and how to open schools was driven more by politics than science. We are left-leaning Boston physicians who have treated numerous covid-19 patients, many of whom — too many — died of the disease. We have been consistent advocates for protecting public health. Now it’s time for those of us who spent 2020 insisting on tight restrictions to recognize that those policies are no longer universally necessary. Today, creating clear guidance for relaxing restrictions — especially rules that could impede more normal school — is what “following the science” calls for.

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