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An unvaccinated elementary school teacher who took off their mask to read to students ended up infecting more than half of them last May – and they went on to infect other students, family members and community members, California public health officials reported Friday.

It’s a prime example of how easy it is to undermine efforts to protect children too young to be vaccinated, US Centers for Disease Control and Prevention Director Dr. Rochelle Walensky said.

The teacher came to work even though they had Covid-19 symptoms and then took off their mask to read to the young students, a team at Marin County Public Health reported in the CDC’s weekly report on death and disease. The teacher assumed the symptoms indicated allergies, not infection, the investigators found.

In the classroom of 22 students, 12 became infected – including eight out of 10 students in the two front rows.

And that’s despite layers of measures intended to prevent transmission of the virus.

“The school required teachers and students to mask while indoors; interviews with parents of infected students suggested that students’ adherence to masking and distancing guidelines in line with CDC recommendations was high in class. However, the teacher was reportedly unmasked on occasions when reading aloud in class,” the report read.

“Throughout this period, all desks were separated by six feet,” it added. “All classrooms had portable high-efficiency particulate air filters and doors and windows were left open.”

It wasn’t enough to protect the kids.

Eventually, 27 people, including the teacher, were infected. None were seriously ill and all recovered. Those cases that were analyzed involved the Delta variant of coronavirus, although the researchers said they were not necessarily able to test everyone who may have been infected in the outbreak.

The CDC highlighted the case as an example of how schools need to follow all recommendations if they want to protect students and staff.

“The introduction of the virus into the classroom by a teacher who worked in school, while she was both symptomatic and unvaccinated and who was unmasked when reading aloud to a class, resulted in cases within the classroom, across the school and among families of students and staff in the community,” Walensky told a White House Covid-19 briefing Friday.

“We know how to protect our kids in school. We have the tools.”

The CDC’s guidance for schools lists vaccination as the No. 1 way to protect everyone. “Vaccination is the leading public health prevention strategy to end the COVID-19 pandemic. Promoting vaccination can help schools safely return to in-person learning as well as extracurricular activities and sports,” it says.

Kids under 12 are not eligible for vaccination, and the CDC says it’s important for the adults around them to get vaccinated to protect them.

Dr. Lisa Santora, deputy health officer for the county, said officials there had been urging teachers to be vaccinated since January, but many had not done it. “We saw firsthand that it wasn’t kids who were going to get teachers sick. It was going to be the reverse,” Santora told CNN.

Santora said Marin County had organized “Super Saturday” events to encourage teachers and staff to get vaccinated, but some teachers still remain unvaccinated. “Adults are underestimating their risk of hospitalization when they are choosing not to get vaccinated,” she told CNN.

She said 90% of people in hospitals with Covid-19 in the county right now are unvaccinated – and many are between the ages of 30 and 50.

The outbreak at the elementary school was a heads up to the county that the Delta variant was going to make it harder to prevent outbreaks, she said.

“We had a few of the adults who were part of the outbreak – they were parents of children in school – they were vaccinated. That was another indication to us that Delta was different,” she said.

“Among the five infected adults, one parent and the teacher were unvaccinated; the others were fully vaccinated,” the report reads. “The vaccinated adults and one unvaccinated adult were symptomatic with fever, chills, cough, headache, and loss of smell. No other school staff members reported becoming ill.”

The county was able to conduct an intensive contact tracing operation that painted a good picture of what happened. Several children appeared to have been infected during a sleepover, and some from the infected teacher’s class appear to have infected siblings.

Testing of most of the children who were exposed was key, because many of the children never developed symptoms.

“The school was on point with all of their mitigation strategies,” Santora said. “I think if it wasn’t Delta, I don’t think we would have seen this.”

Nonetheless, the CDC concluded, schools can safely open for in person classes if they take care.

Los Angeles County officials studied cases in their schools from September to March. They counted 463 cases among students in that time that could be linked back to a school exposure, and 3,927 among staff who were back in person. This was a far lower case rate than in the community at large during the same period, they reported.

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“In schools with safety protocols in place for prevention and containment, case rates in children and adolescents were 3.4 times lower during the winter peak compared with rates in the community,” they wrote.

“This analysis reflects transmission patterns before the more transmissible SARS-CoV-2 B.1.617.2 (Delta) variant became predominant in the United States. A multipronged prevention strategy, including masking, physical distancing, testing, and most recently vaccination of children and adolescents aged 12 years and above, will remain critical to reducing transmission as more students return to the classroom,” they added.

“These findings from a large and diverse county present preliminary evidence that schools provided a relatively safe environment during the 2020-21 school year.”