A COVID-19 delta variant outbreak at an elementary school in Marin County originated with a single unvaccinated teacher who spread the disease to half the students in the classroom and beyond, according to a CDC study released Friday.
While ill, the teacher read aloud to the class without a mask, in violation of local policy, the Centers for Disease Control and Prevention study authors reported.
By the end of the outbreak, health officials located 27 positive cases, and 22 of those had symptoms. No one was hospitalized.
The students in the teacher’s classroom — and another class nearby where students were also sickened — were too young to be vaccinated.
The outbreak occurred in May at an unidentified 205-student school that serves pre-K through eighth grade. All but two of the school’s staff members were vaccinated, the CDC said.
The teacher was not named.
The report’s findings ultimately explain why it’s important for school staff members to get vaccinated — and adhere to safety protocols such as masking — since they are in close indoor contact with children who are still ineligible for the COVID-19 vaccine, KTLA sister station KRON in San Francisco reported.
The study, published in the CDC’s Morbidity and Mortality Weekly Report, was produced by a Marin County health official and researchers from UC Berkeley, UC Davis and UC Santa Cruz.
The CDC described the chain of events at the school:
- May 13-16: Teacher attends social events, does not report any known COVID-19 exposure
- May 17-21: Teacher goes to work in the classroom, at some point reads aloud to the class while unmasked
- May 19: Teacher becomes symptomatic with nasal congestion and fatigue, attributes these to allergies; symptoms come to include cough, subjective fever and headache
- May 21: Teacher gets a coronavirus test
- May 22-26: Teacher’s students begin experiencing symptoms
- May 22-31: Students in another nearby classroom begin experiencing symptoms
- May 23: Teacher reports positive test result to school, self-isolates through May 30, recovering during that time
- May 23-26: Twenty-two of the teacher’s 24 students are tested; 12 test positive
- May 26, June 2: Testing events are held at the school; further cases discovered
Masks were required indoors at the school, and all classrooms had “portable high-efficiency particulate air filters,” the CDC said. Classroom doors and windows were left open.
Nonetheless, the teacher was reported to have read aloud to the classroom without wearing a mask.
In the two rows seated closest to the teacher’s desk, 80% of the students later tested positive. The rate was 21% in the three back rows, the CDC found.
That “attack rate” highlights the delta variant’s “increased transmissibility and potential for rapid spread,” especially among unvaccinated children, the study’s authors said.
Here’s a map of the classroom where the outbreak started, which the CDC included in its report:
In the other classroom thought to be part of the same outbreak, 14 of the 18 students were tested — and six came back positive.
That second classroom, which was for students in a different grade and across a large outdoor courtyard from the teacher’s classroom, included one student who held a May 21 sleepover with two other classmates; all three of the children tested positive.
It’s not clear how the infection spread from the one classroom to the other, but the epidemiologic link is “thought to be interaction at the school,” the study said.
Ultimately four other students in other grades tested positive. They were all siblings of students in the teacher’s class, and the CDC said “exposure was assumed to have occurred in their respective homes.”
Four parents, one of whom was unvaccinated, also became infected.
Health officials hosted testing events shortly after the classroom results. That testing included 231 people — 194 students, 21 staff members and 16 students’ family members.
The study 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, according to the study.