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

What is already known about this topic? The SARS-CoV-2 B.1.617.2 (Delta) variant is highly transmissible. Prevention guidance in schools varies by jurisdiction. What is added by this report? During May 23–June 12, 2021, 26 laboratory-confirmed COVID-19 cases occurred among Marin County, California, elementary school students and their contacts following exposure to an unvaccinated infected teacher. The attack rate in one affected classroom was 50%; risk correlated with seating proximity to the teacher. What are the implications for public health practice? Vaccines are effective against the Delta variant, but transmission risk remains elevated among unvaccinated persons in schools. In addition to vaccination, strict adherence to multiple nonpharmaceutical prevention strategies, including masking, are important to ensure safe school instruction.