Outbreak Associated With SARS-CoV-2 B.1.617.2 (Delta) Variant in an Elementary School

Marin County, California, May-June 2021

Tracy Lam-Hine, MBA; Stephen A. McCurdy, MD; Lisa Santora, MD; Lael Duncan, MD; Russell Corbett-Detig, PhD; Beatrix Kapusinszky, PhD; Matthew Willis, MD


Morbidity and Mortality Weekly Report. 2021;70(35):1214-1219. 

In This Article

Investigation and Findings

The outbreak location was an elementary school in Marin County, California, which serves 205 students in prekindergarten through eighth grade and has 24 staff members. Each grade includes 20 to 25 students in single classrooms. Other than two teachers, one of whom was the index patient, all school staff members were vaccinated (verified in California's Immunization Registry). The index patient became symptomatic on May 19 with nasal congestion and fatigue. This teacher reported attending social events during May 13–16 but did not report any known COVID-19 exposures and attributed symptoms to allergies. The teacher continued working during May 17–21, subsequently experiencing cough, subjective fever, and headache. 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. However, the teacher was reportedly unmasked on occasions when reading aloud in class. On May 23, the teacher notified the school that they received a positive result for a SARS-CoV-2 test performed on May 21 and self-isolated until May 30. The teacher did not receive a second COVID-19 test, but reported fully recovering during isolation.

The index patient's students began experiencing symptoms on May 22. During May 23–26, among 24 students in this grade, 22 were tested. A COVID-19 case was defined as a positive SARS-CoV-2 reverse transcription–polymerase chain reaction (RT-PCR) or antigen test result.* Twelve (55%) of the 22 students received a positive test result, including eight who experienced symptom onset during May 22–26. Throughout this period, all desks were separated by 6 ft. Students were seated in five rows; the attack rate in the two rows seated closest to the teacher's desk was 80% (eight of 10) and was 28% (four of 14) in the three back rows (Fisher's exact test; p = 0.036) (Figure 1).

Figure 1.

Classroom layout and seating chart for 24 students in index patient's class, by SARS-CoV-2 testing date, result or status, and symptoms — Marin County, California, May–June 2021

On May 22, students in a another classroom, who differed in age by 3 years from the students in the class with the index case and who were also ineligible for vaccination began to experience symptoms. The two classrooms were separated by a large outdoor courtyard with lunch tables that were blocked off from use with yellow tape. All classrooms had portable high-efficiency particulate air filters and doors and windows were left open. Fourteen of 18 students in this separate grade received testing; six tests had positive results. Investigation revealed that one student in this grade hosted a sleepover on May 21 with two classmates from the same grade. All three of these students experienced symptoms after the sleepover and received positive SARS-CoV-2 test results. Among infected students in this class, test dates ranged from May 24 to June 1; symptom onset occurred during May 22–31.

In addition to the documented infections in the two initial grades, cases were identified in one student each from four other grades. Three patients were symptomatic; dates for testing were May 30 or June 2. These four students were siblings of three students with cases in the index patient's class, and exposure was assumed to have occurred in their respective homes. In addition to the teacher and 22 infected students, four parents of students with cases were also infected, for a total of 27 cases (23 confirmed by RT-PCR and four by antigen testing) (Figure 2). Among the five infected adults, one parent and the teacher were unvaccinated; the others were fully vaccinated. The vaccinated adults and one unvaccinated adult were symptomatic with fever, chills, cough, headache, and loss of smell. No other school staff members reported becoming ill. No persons infected in this outbreak were hospitalized. This activity was reviewed by Marin County and was conducted consistent with applicable law.

Figure 2.

Timeline of SARS-CoV-2 illness onset* after onset in the index patient (A) and presumed transmission pathway (B) among students, siblings, and parents, relative to onset in the index patient — Marin County, California, May 2021
*Symptom onset date or specimen collection date, if asymptomatic.
Presumed transmission based on phylogenetic and epidemiologic analyses.