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


This outbreak of COVID-19 that originated with an unvaccinated teacher highlights the importance of vaccinating school staff members who are in close indoor contact with children ineligible for vaccination as schools reopen. The outbreak's attack rate highlights the Delta variant's increased transmissibility**and potential for rapid spread, especially in unvaccinated populations such as schoolchildren too young for vaccination. However, transmission to community contacts appeared lower than that of some previously reported Delta variant outbreaks.[5] Further transmission might have been prevented by high levels of community vaccination; at the time of this outbreak, approximately 72% of eligible persons in the city where the school is located were fully vaccinated.†† These findings support evidence that the current COVID-19 vaccines with Food and Drug Administration approval or Emergency Use Authorization are effective against the Delta variant; however, transmission risk remains elevated among unvaccinated persons in schools. In addition to vaccination of eligible persons, implementation of and strict adherence to multipronged nonpharmaceutical prevention strategies including proper masking, routine testing, ventilation, and staying home while symptomatic are important to ensure safe school instruction.

The findings in this study are subject to at least three limitations. First, the teacher's specimen was unavailable for WGS, which prevented phylogenetic identification of the outbreak's index patient. Second, testing for parents and siblings was self-directed and took place mostly outside the school setting, which could have led to underascertainment of cases. Finally, challenges in testing acceptance among possible contacts from outside the school led to difficulty in characterizing the outbreak's actual spread into the community, as is evidenced by later discovery of additional community cases with sequences indistinguishable from those in the school outbreak.

Ineligibility because of age and lack of vaccination contribute to persistent elevated risk for outbreaks in schools, especially as new SARS-CoV-2 variants emerge. However, implementation of multiple prevention strategies within schools can mitigate this risk. The rapid transmission and vaccine breakthrough infections in this outbreak might have resulted from the schoolchildren's vulnerability because of ineligibility for vaccination, coupled with the high transmissibility of the Delta variant. New evidence of the Delta variant's high transmissibility, even among fully vaccinated persons,[6,7] supports recommendations for universal masking in schools§§.[1] Further application of nonpharmaceutical prevention strategies, including routine testing, ventilation, and staying home while symptomatic, are also important for protecting the health of schoolchildren ineligible for vaccination because of their age.[3] In addition, phylogenetic analysis can help to clarify transmission patterns and characterize outbreak progression. Capacity-building efforts offered by regional and state laboratories enabled more sophisticated analysis at the local level; such efforts might be useful as vaccination rates increase, new variants emerge, and outbreaks become more localized.