NOVATO (KPIX) — An in-depth report from the Centers for Disease Control highlighting a major COVID-19 outbreak at one Marin County school is giving a clear picture of the do’s and don’ts for schools that are fully reopening.

The outbreak happened at Our Lady of Loretto School in Novato, in May 2021. The new details of how an unvaccinated teacher spread the virus unintentionally, have bigger implications for all school communities.

READ MORE: COVID: Unvaccinated Marin County Teacher Infected Students in May Delta Variant Outbreak

One unvaccinated teacher, showing symptoms, continued to work at the school anyway for two days before getting a test, and infected least 26 others including students, parents, and siblings.

“I think it caused the school a lot of distress and the students, as well,” said parent Annie Smith.

A seating chart detailed in the CDC report shows how the unnamed teacher in late May, quickly spread the virus to half of the classroom, infecting 8 out of 10 students in the first two rows.

“This is is our largest outbreak in the county that we have investigated to this day,” said Dr. Lisa Santora of Marin Health and Human Services.

The teacher read aloud to students, but did not wear a mask.

READ MORE: COVID: Bay Area Health Officers Reaffirm In-Person Schooling Amid Delta Variant Concerns

Dr. Lisa Santora is one of the authors of the report, based on more than 230 on-campus tests, contact tracing, and whole genome sequencing, showing the highly transmissible Delta variant even in vaccinated adults.

“With the Alpha variant if you were sick you might get one or two other people sick with the Delta variant we see the attack rate between five and six people,” said Dr. Santora.

It’s the perfect example of how the combination of unvaccinated and no mask can lead to widespread in-school transmission that’s preventable.

The layered protections including masking, staying home when symptomatic, and vaccinations when eligible, can keep schools open and safe for in-person learning.

“It was pretty crazy but we’re happy that they’re back in school now and can experience back to being in person,” said Dr. Santora.

Health experts say if this can happen in Marin County, where it’s highly vaccinated, the consequences can be that much worse in widely unvaccinated communities across the country.

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KPIX reached out to the school and archdiocese of San Francisco which oversees the parochial schools in Marin County, but so far, there has been no response.