An unvaccinated teacher who showed up to her elementary school classroom in Marin County, California, while experiencing symptoms such as "cough, subjective fever, and headache" ended up infecting half of her students with Covid-19 as well as some of their family members, according to a report from the Centers for Disease Control and Prevention released Friday.
Officials from the Marin County Department of Public Health initiated an investigation into the classroom outbreak on May 26, three days after the teacher reported testing positive for Covid-19. The teacher, who initially attributed symptoms to allergies, was one of only two staff members that had not been vaccinated at the elementary school.
Among the teacher’s 24 students, 22 who were ineligible for vaccination because of age were tested. Twelve tested positive for the virus, according to the report, written by county health officials and experts at the University of California's Berkeley, Davis and Santa Cruz campuses.
Children under 12 are ineligible for vaccination, meaning that their safety is dependent on others adults to get vaccinated to minimize exposure to the virus, according to the CDC.
Experts concluded that the attack rate in the affected classroom was 50 percent, but the students seating closest to the teacher's desk faced an increased risk of infection. The attack rate in the two rows seated closest to the teacher’s desk was 80 percent, the CDC reported.
A majority of the students sitting in the first two rows of the classroom contracted the virus, while a minority of those sitting at the back of the class were infected. The teacher occasionally read aloud to students while unmasked, despite school requirements to mask while indoors, according to the report.
Four students from other classrooms also tested positive to Covid-19. They were all siblings of three students in the unvaccinated teacher's classroom, "and exposure was assumed to have occurred in their respective homes," the CDC reported.
Four parents of children at the school were later infected in the outbreak, according to the report. Of the infected parents, only one was unvaccinated. The vaccinated experienced symptoms including fever, chills, cough, headache and loss of smell.
Additionally, six students from a different grade also tested positive for Covid-19 after one student hosted a sleepover with two other students from the same class, the CDC reported. All students infected in this classroom were also ineligible for vaccination due to age.
Certain specimen evidence gathered throughout the investigation suggests "that infections occurring in the two grades likely were part of the same outbreak."
A total of 26 elementary school students and their contacts got infected following exposure to the unvaccinated teacher. At least 18 of them contracted the delta variant, the CDC concluded.
"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," the report reads.
Experts also concluded that the outbreak’s impact on the broader community may have been limited thanks to Marin County’s high vaccination rate, one of the best in California. At the time of the outbreak, 72 percent of eligible people had been fully vaccinated in the city where the school is located, according to the report.
However, new evidence of the delta variant’s "high transmissibility, even among fully vaccinated persons, supports recommendations for universal masking in schools. ... Proper masking, routine testing, ventilation, and staying home while symptomatic are important to ensure safe school instruction," the CDC said.