MARIN COUNTY, Calif. (KRON) – The Centers for Disease Control and Prevention on Friday released a study into how a COVID-19 delta variant outbreak at an elementary school in Marin County, California, originated with a single unvaccinated teacher.

The report’s findings ultimately explain why it is important for school staff members to get vaccinated against COVID-19 since they are in close indoor contact with children who are ineligible for the COVID-19 vaccine at this time.

The Pfizer vaccine is the only one approved for children 12 years of age and older, and it has also recently been fully approved by the U.S. FDA.

The elementary school has 205 students enrolled, from prekindergarten through eighth grade, with 24 staff members working onsite. The CDC reports that all school staff, except for the original COVID-19 patient and one other teacher, were fully vaccinated. By the end of the outbreak, health officials would locate 27 positive cases.

Safety Measures

The report found that despite using safety measures like separating the desks by 6 feet and wearing face masks, the teacher had transmitted the COVID-19 delta variant to 12 of the 22 students that were tested.

“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 (3) was high in class,” the report said.

However, the teacher was reported to have read aloud to the class while unmasked on some occasions.

The attack rate — or percentage of the healthy proportion who become ill — was highest close to the teacher. In the two rows seated closest to the teacher’s desk, 80% (eight of 10), were positive. The rate was 21% (three of 14) in the three back rows, the CDC found.

Here’s a map of the classroom where the outbreak started, which the CDC included in its report:

The blue-shaded squares indicate the teacher, up top, and the students who tested positive and also became symptomatic.

The squares outlined in blue show students who tested positive but were asymptomatic.


The unvaccinated teacher first became symptomatic on May 19 — and reported feeling nasal congestion and fatigue and attributed this to allergies. The teacher was working from May 17-21, and began showing the following symptoms: cough, subjective fever, and headache.

At this point, the teacher got a COVID-19 test on May 21, and then reported to the school on May 23 that the test came back positive. The teacher then self-isolated until May 30 and fully recovered.

The CDC reports that the students began showing symptoms on May 22. Twenty-two out of the 24 students were tested between May 23-26, and that’s when 12 came back with a positive result.

Outbreak Outside the Classroom

The COVID-19 delta variant was not isolated to the singular classroom, however. Another classroom with students who had a 3-year age gap from the original classroom began showing symptoms on May 22. The two classrooms were separated by a large outdoor courtyard.

This classroom had 18 students, 14 of which got tested. From there, six students received positive test results.

Health officials also hosted testing events shortly after the classroom results, during which 231 people were tested for COVID-19. This included 194 of 205 students, 21 of 24 staff members and teachers, and 16 parents and siblings of students.

A laboratory looked deeper into the collected specimens with the detectable virus. First, they confirmed that all cases were the delta variant.

They then found that 11 of the sequences had no genetic differences. Here’s who they came from:

  • Six of these indistinguishable sequences came from students in the original classroom
  • Four of them were from students in a separate grade
  • One was from a sibling of a student in the original classroom.

According to the CDC, this suggests that the infections in the two different grade levels likely were part of the same outbreak.

“The epidemiologic link between the two grades remains unknown but is thought to be interaction at the school,” the report says.

The CDC also believes the outbreak infected four total siblings of different students in the original classroom and three total parents of different students in the original classroom.

The CDC said community transmission appeared lower than previous outbreaks they have studied, and said it might be due to a high level of community vaccination: 72% of eligible people in the city where the school is located were fully vaccinated, the CDC said.

What Now?

The CDC recommends for all eligible people to get vaccinated, especially teachers who are in close contact with ineligible students. They also recommend staying home if showing symptoms, getting tested routinely, and wearing face masks inside,.