An unvaccinated primary-school teacher in Marin County, California, spread COVID-19 to 26 other people, including 50% of their classroom, after spending two days sick with the coronavirus at school while not always masking, according to a new Centers for Disease Control and Prevention report.

The report, published Friday, highlights how vital it is to both vaccinate and mask elementary-school teachers to protect children under 12, who cannot get vaccinated yet.

"We know how to protect our kids in school. We have the tools," CDC Director Rochelle Walensky said Friday, discussing the report during a White House COVID-19 briefing.

The teacher in question "was both symptomatic and unvaccinated," Walensky said, and "was unmasked when reading aloud to the class," whose students were all too young to be vaccinated.

The teacher's actions resulted in 12 COVID-19 cases among the class of 24 kids, along with six other illnesses at the K-8 school, and eight parents and siblings were infected, the report said.

The investigation used viral-genome sequencing to determine the cases were all likely related. All the COVID-19 cases sequenced in the outbreak were classified as the Delta variant, which is about twice as contagious as other versions of the virus.

No one was hospitalized.

The unvaccinated teacher spread COVID-19 to 12 of their 24 students, despite distancing and masks on the kids, the CDC said.

The CDC drew up a map of the classroom, which shows the prevention measures that had been taken.

Students' desks were spaced apart 6 feet, windows and doors were left open to promote good ventilation, and an air filter was installed at the head of the class. Kids' adherence to CDC guidelines on both masking and distancing in school was "high" in class, according to interviews the CDC conducted with parents.

But with an unmasked, unvaccinated teacher at the helm, none of that appeared to matter so much.

The teacher continued teaching for two days while symptomatic, starting on May 19, with their symptoms worsening from initial congestion and fatigue that they wrote off as allergies to a cough, fever, and headache.

It did seem that sitting farther away from the sick teacher helped protect students, which makes sense when you consider how the coronavirus travels from person to person through the air when sick people talk or shout.

"The attack rate in the two rows seated closest to the teacher's desk was 80% (8 of 10) and was 21% (3 of 14) in the three back rows," the CDC report said.

The finding underscores what other experts have been saying for a while now — more adults need to be vaccinated to protect everyone from the Delta variant.

"Six feet is not magical," Mike Osterholm, the director of the Center for Infectious Disease Research and Policy at the University of Minnesota, said Thursday on his podcast, stressing that he thought the CDC guidelines didn't go far enough to prevent the spread of the more contagious Delta variant.

"We can make [school] safer, and we have to do that," Osterholm said. "The first thing we do is we use our vaccines."