An elementary school in Marin County experienced an outbreak of the COVID-19 delta variant from May 23 through June 12.
The outbreak was associated with an unvaccinated teacher who, despite a mask policy at the school, was occasionally unmasked while reading aloud to the class, according to a Centers for Disease Control and Prevention, or CDC, report. 12 of the 24 students in the teacher’s classroom tested positive for COVID-19, and there was an 80% infection rate among students seated in the rows closest to the teacher.
In another class, six of 18 students tested positive for COVID-19. The virus was transmitted to eight other confirmed community members, including siblings and parents, according to the report.
None of the cases associated with the outbreak required hospitalization and everyone has since recovered, noted Tracy Lam-Hine, a doctoral candidate at the UC Berkeley School of Public Health and epidemiologist with Marin County Health and Human Services.
“I do feel that it is possible for schools to open safely, and this is highly desirable for all concerned,” said Stephen McCurdy, UC Davis professor emeritus in the department of public health sciences and department of internal medicine, in an email. “However, it requires sustained and consistent attention to all the preventive measures at our disposal.”
Each school in Marin County has a public health liaison who attends weekly trainings with Marin County Public Health. They are required to notify the county of all positive cases among students and staff, according to Lisa Santora, Marin County deputy public health officer.
From there, the county can make decisions about who needs to be quarantined and whether there should be classroom closures, according to Matt Willis, Marin County public health officer.
The initial outbreak was tracked using contact tracing and genome sequencing. Those who had been in contact with people who tested positive were interviewed in order to find others who were potentially exposed to the virus, according to McCurdy.
The Marin County Public Health team provided exposed students with rapid testing and sent confirmatory tests for PCR and sequencing, according to Santora.
“(Genome sequencing) is essentially like a family tree of the virus,” Lam-Hine said. “We can see where there’s genetically identical virus, which leads us to believe that we can make stronger assumptions about who infected who and when the actual transmission occurred.”
According to Willis, the county had 12 school-based COVID-19 cases total in the eight months prior to the outbreak.
Russell Corbett-Detig, UC Santa Cruz professor of biomolecular engineering, noted that Marin County has a very high vaccination rate among the eligible population.
“We can be thankful that this happened when Marin had a very high level of vaccination,” McCurdy said in an email. “Had it occurred prior to the advent of the vaccine, it is highly likely it would have exploded.”
McCurdy emphasized that he is not blaming any individual, but is instead stressing the importance of safety policies, particularly those for children who are not eligible for vaccination.
Willis added outbreaks like the one in Marin County can act as learning opportunities for practices and policies, and that it is important to look at the bigger implications.
“With delta, it is so infectious and the viral load is so much higher that we can’t let our guard down,” Lam-Hine said. “We need vaccination, we need masks, we need routine testing and we need ventilation.”