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Outbreak Associated with SARS-CoV-2 B.1.617.2 (Delta) Variant in an Elementary School — Marin County, California, May–June 2021

Our take —

An outbreak of 26 laboratory-confirmed Delta-variant SARS-CoV-2 cases took place at an elementary school in Marin County, California in May 2021 following exposure to an unvaccinated infected teacher. Despite school guidelines requiring mask wearing at all times, the teacher removed her mask to read aloud at times. While it is not possible to confirm that some/all of the subsequent infections were a result of exposure to this index patient, the attack rate in the homeroom classroom among students was high (54%), and even higher for the students sitting in the first few rows of the classroom closest to the teacher (80%). The total number of secondary cases outside of students in direct contact was relatively low and no cases were hospitalized, which may be partially explained by the high levels of community vaccination among eligible adults including parents and other teachers. These results, however, highlight the potential risk of infection to children who are ineligible for vaccination and their families (including vaccinated members) when COVID-19 prevention measures are not fully adopted by schools.

Study design

Case Series

Study population and setting

An outbreak investigation at an elementary school (pre-K to 8th grade) in Marin County, California was conducted by the Marin County Department of Health following a notification that an unvaccinated teacher had tested positive for SARS-CoV-2. The outbreak investigation involved contact tracing and whole genome sequencing of available specimens. The teacher (index patient) first started experiencing symptoms on May 19, continued working through May 21, and reported receiving a positive test to the school on May 23 (performed on May 21). School regulations required desks to be separated by 6 feet at all times; classrooms were equipped with particulate air filters; windows and doors were kept open as much as possible. Students and teachers during this period were required to be masked; it was reported that the index patient removed her mask at times to read aloud.  

Summary of Main Findings

During the outbreak investigation, 27 cases (including the index) were identified. Out of the teacher’s 24 students, 22 (ineligible for the vaccine) received testing and 12 tested positive (12/22; 54%). The attack rate among students was higher in the two rows closest to the teacher (8/10; 80%) compared with the back three rows (4/14; 28%), (Fisher’s exact test: p 0.036). Students in an additional classroom, separated from the original classroom by a courtyard, reported experiencing symptoms and 14/18 received testing; 6 of the 14 tested in this additional class tested positive (43%). Additional cases were identified among students in other grades (n=4) and among parents (n=4). The additional 8 cases were siblings and parents of students in the index patient’s class. Of those eligible for vaccination (index patient and parents), the teacher and one parent were unvaccinated. No one was hospitalized. All sequenced samples (n=18) were identified as the B.1.617.2 (Delta) variant.

Study Strengths

Given that most of the students who were exposed received testing, this outbreak investigation provides an important data estimate of secondary attack rate during the early days of the surge of the Delta variant within a school-setting in which precautions were taken, but teachers were not fully vaccinated.


It is not possible to say with absolute certainty that the unvaccinated teacher was the source of any of the subsequent infections (the teacher’s sample was not available for genetic sequencing and so phylogenetic tracing was not possible). Additionally, the ascertainment of cases outside of the students may be limited as parents and siblings had to opt-in. 

Value added

This investigation adds evidence on the importance of 1) vaccination, 2) staying home when symptomatic, 3) masking, and 4) timely contact tracing all within a school setting.  

This review was posted on: 11 September 2021