Health authorities in Marin County, California, say an unvaccinated and sometimes unmasked teacher likely caused a coronavirus outbreak at an elementary school earlier this year.
A report from health officials said the teacher tested positive with the Delta variant in late May. It said 26 others, students and parents, also tested positive by early June. The school has about 200 pupils.
The document said the teacher continued to work for two days until receiving a test, even after feeling symptomatic.
The report said the teacher occasionally read aloud, unmasked to the class, despite the school's requirement that teachers wear masks while indoors.
People in the US are paying closer attention to anti-infection measures at schools as fall terms get underway.
The Centers for Disease Control and Prevention is stressing the importance of vaccination and other anti-virus measures, such as mask wearing and adequate ventilation, to prevent infection among children too young to receive an inoculation.