The CDC has just released a study focused on how one unvaccinated teacher infected with the Delta variant of COVID-19, in a mostly masked Marin County classroom, managed to infect half their students and seeded an outbreak that hit another classroom and students' families as well.

In a case that illustrates both how essential vaccine mandates are for schoolteachers and how contagious the Delta variant can be in an indoor classroom setting with unvaccinated students, the Marin outbreak holds lessons that should be learned nationwide.

The outbreak occurred in late May, as KRON 4 reports, before many of us understood how easily the Delta variant could be spread, and neither the school nor the city it's in were identified in the CDC report — however we know from Marin County health officials that a Delta variant outbreak occurred around that time at a school in Novato.

As the CDC explains, the unvaccinated teacher was infected sometime in mid-May 2021, and became symptomatic on May 19 with a stuffy nose and fatigue. The teacher was in the classroom with students from May 17 to May 21, and tested positive for COVID-19 on May 21. The teacher's subsequent symptoms included cough, fever, and headache.

Much like an early study out of China that illustrated how the airborne coronavirus spread in a restaurant, the CDC study provides a map of where the teacher's desk was located in relation to students' desk, and where each of the infected students sat. Students in the class, included many in the front two rows of the classroom, began showing symptoms on May 23 and 24, and subsequently tested positive. All told, 12 out of 24 students in the class were infected and tested positive, including several in the back rows of the class. The infection rate for the front two rows of the class was 80%.

These transmissions occurred despite students adhering to mask protocols, the presence of an air filter at the front of the class, six feet of distancing between desks, windows left open on both sides of the classroom, and the door of the classroom being kept open.

Figure via CDC

Students reported that the teacher was mostly masked, however the teacher removed their mask to read aloud to the class on multiple occasions.

"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," the report says.

Figure via CDC

Additionally, due to likely contacts at school as well as contacts with siblings at home, the infected students spread the virus to at least six other students in another classroom who were three years different in age from those in the original classroom. The two classrooms were separated by a large outdoor courtyard, and out of a class of 18, 14 students were tested, and six came back positive.

A lab was able to confirm that all of the cases were Delta variant cases, and that at least 11 of the virus samples taken were indistinguishable from each other, confirming that the infected teacher was likely responsible for all the subsequent cases.

The outbreak also spread to four siblings of four different students in the original classroom, as well as to three total parents of three different students in the original classroom.

The school reported that all of its teachers and staff were vaccinated at the time of the outbreak, save for this one teacher and one other.

The CDC noted that there appeared to be less community transmission from this outbreak than in other outbreaks the agency has studied, likely because of high overall vaccination in the community when this outbreak occurred. At the time of the outbreak, the CDC said the city in which it occurred had a 72% rate of vaccination.

At the time of this outbreak, another occurred in Marin County that was centered in San Anselmo and Fairfax. Health Officer Dr. Matt Willis said at the time, because both outbreaks lasted only two weeks and appeared to be stopped short by the vaccines, "This is what herd immunity looks like."