How often is COVID-19 spreading in Massachusetts schools?

Karel Lindor (left) waited in line with his son Khamani at Milton High School for testing of any Milton Public School student and/or family members who may have been exposed to the coronavirus.Jonathan Wiggs/Globe Staff

When Governor Charlie Baker urged superintendents to keep classrooms open, he left a gut-wrenching question unanswered: How often is coronavirus spreading in schools?

Baker says most schools should only close if they have seen in-school transmission, which he suggests is rare — an assumption seemingly bolstered by national reports of low rates of infection among those who have returned to school.

Yet superintendents have publicly reported spread within their schools. Skeptical teachers’ unions have created a database to self-report cases across the state. And school employees have been sharing harrowing hospital stories. The state epidemiologist acknowledges that while the exact rate is unclear, in-school transmission is, in fact, occurring.

“There’s no question there is, and has been, in-school transmission,” said Dr. Catherine Brown, state epidemiologist for the Massachusetts Department of Public Health. “There is so much COVID out here in the communities that people are getting exposed and they’re bringing COVID without knowing it into the school settings.”

In the pitched debate over how to get students back to school, given the mounting consensus that remote learning is harming kids’ academic and social emotional growth, nuance is often lost. Teachers’ unions seem to suggest aging school buildings are dangerous unless proven otherwise. Politicians and some public health specialists promote schools as if they are safety zones from a pandemic.

“It’s not a magic bubble location,” said Emily Oster, a Brown University economist who has built one of the country’s most comprehensive databases of school COVID cases and found surprisingly little spread.

“People are craving certainty,” Oster acknowledged. “When we say schools are very safe, [people] want to read it as ‘schools are totally safe with no transmission.’ "

As with all things COVID, the accumulating data present a more complicated risk-benefit analysis.

It’s unclear precisely how many people have been infected in schools, but state officials believe they know the maximum number possible, and they see no cause for alarm.

This school year, they have identified 75 clusters in Massachusetts schools — defined as two or more cases between unrelated people without another clear source of infection. But they think only about half of those clusters could have been linked to in-school transmission, given the timing of infections.

Even if all 208 cases associated with those clusters were caused by in-school spread, however, they would make up just a sliver of the nearly 3,000 COVID cases among students and staffers reported this year. That means the vast majority of infected kids and teachers contracted the virus at home or in the community, not at school.

A kindergarten teacher used her bitmoji to spread awareness about COVID-19 on the door of her classroom asking students to, "Spread Hope, Not Germs!" at Mary L. Fonseca Elementary School in Fall River, where schools remained open despite high COVID rates. (Jessica Rinaldi/Globe Staff) Jessica Rinaldi/Globe Staff

State officials say they’re also reassured that the rate of infection among students and staff attending schools in person is only one-quarter of the infection rate in the community statewide.

For any individual student, the data shows that the risk of in-school transmission is vanishingly small, Baker’s health secretary, Marylou Sudders, said. Just 0.02 percent of the estimated 450,000 students who have been in schools at some point this year have been part of a suspected cluster.

The numbers game is not much comfort to teachers, though, or to parents worried about their children’s risk of contracting COVID at school.

“Our members who have the highest risk factors or live with family members with risk factors are terrified,” said Merrie Najimy, president of the Massachusetts Teachers Association. “They feel like sacrificial lambs.”

Teachers don’t trust the state or national data denying many outbreaks being linked to schools, Najimy said, in part because many places don’t have widespread testing available in schools or communities, so there is no way to know the true rates of COVID including asymptomatic cases, which are more common in children.

That’s a valid concern, said Dr. Leana Wen, the former Baltimore health commissioner and a public health professor at George Washington University.

“There has been a failure of national policy and we have not done right by our children for sure in this pandemic,” Wen said. “But why should the effect of that and the failures of our society all be on the shoulders of the people who work in the schools?”

Rather than wait for a confirmed outbreak to close schools, Wen said, leaders should require schools to demonstrate they have proper preventative safety measures in place, including the ability to socially distance everyone by 6 feet and proper air filtration and ventilation, before being allowed to open.

Many schools have found ways to keep teachers safe — and if all measures have been adhered to, teachers should not have reason to worry if someone in their class tests positive, said Erin Bromage, an immunology professor at University of Massachusetts Dartmouth who has advised school leaders in Rhode Island, Boston, Westport, Dartmouth, and Fairhaven. Small adjustments — like resisting the instinct to walk the aisles of the classroom to help students side-by-side — can make in-person instruction safer.

Public health officials are confident that schools aren’t super-spreaders, even as they acknowledge they are not likely detecting all cases of in-school spread. When a student or staffer tests positive, investigators try to identify any “close contacts,” people who came within 6 feet of the person for 15 minutes or longer while they were likely infectious. Health authorities order those close contacts to quarantine and seek testing, using the results to determine whether they believe transmission occurred in school. Given schools’ distancing measures, many school cases have yielded few “close contacts,” if any, though some situations arise where distancing is difficult such as in hallways, sports, buses, and special education.

In Norwood, for example, no close contacts of school cases so far have tested positive, leading officials there to conclude they haven’t had in-school transmission.

“Can I say for sure that there is in-school transmission or not in my community? No, but at this point, I don’t believe so,” said Sigalle Reiss, director of Norwood’s board of health and president of the Massachusetts Health Officers Association. “There’s never 100 percent [certainty] — we deal with probability a lot.”

The state’s mobile rapid response testing unit has at least 15 times responded to school concerns about outbreaks by offering tests to a larger group of possibly exposed students and staffers in schools including Braintree, Malden, and Billerica. (All 221 people tested in Billerica were negative, indicating no in-school spread.)

Public health or school nurses looking for causes of school infections often find the likely source of transmission at home. For instance, in Westport, a small South Coast district of 1,355 students, nurses have been tracking reports whenever students call out sick or quarantine due to relatives’ diagnoses, “so we’ve been able to identify all of our cases so far,” said Diane Daponte, one of two lead nurses for the district.

More than two dozen cases of COVID have been reported in Westport schools since the primary and elementary schools opened full time this fall. But in-school transmission was only suspected once, because two students from the same classroom tested positive, within days and had no obvious sources of infection outside school. As a result, the classroom was closed for deep-cleaning and a two-week quarantine.

Oster said that more detailed information about in-school transmission would help identify and avert specific risks — for instance, if outbreaks are occurring in faculty lounges or locker rooms — thereby putting workers’ minds at ease and preventing wholesale closures.

Teachers are also suspicious of the state’s numbers, as many COVID cases they learn about aren’t publicly reported. The unions launched an online database in mid-November to crowd-source reports of COVID cases, in-school spread, and other safety issues, amid the governor’s assertions that they should be back in school.

“It makes me very angry,” said one Milton Public Schools educator who is “1,000 percent” sure she contracted coronavirus in school in October, when she was identified as a “close contact” of a co-worker. They were the first ones in their families to contract the virus, at about the same time as four other employees and one student in school tested positive.

Her co-worker, who has since recovered, had initially gone to the emergency room to be screened for pneumonia, only to be whisked away by nurses warding off passer-by by warning “positive COVID.”

After three nights in the hospital and a course of remdesivir, she recovered, and returned to work at school, further humbled by a virus that she had tried to guard against with a mask and a plexiglass cube around her desk.

“It’s no joke,” she said.