MARIN COUNTY (CBS SF) — An unvaccinated elementary school teacher in Marin County appears to have been at the center of a COVID delta variant outbreak last May, according to a new report released by the Centers for Disease Control and Prevention.

The report that was released by the CDC on Friday stated that back on May 25, the Marin County Department of Public Health was informed by an elementary school that two days earlier, an elementary school teacher who had not been vaccinated reported receiving a positive COVID-19 test result.

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According to the report from the CDC, the teacher reported becoming symptomatic on May 19, but continued to work for two more days prior to receiving a test on May 21. During those two days working at the school, the teacher read aloud unmasked inside the classroom despite a school requirement to remain masked while indoors.

Beginning on May 23 the same day, the teacher received the positive COVID-19 test result — additional cases were reported among other staff members, students, parents and siblings connected to the school. The Marin County Department of Public Health initiated an investigation with contact tracing that identified a total 27 cases including the unvaccinated teacher.

According to the CDC, during May 23–26, 22 of the teacher’s 24 students — all of whom were ineligible for COVID vaccination due to their young age — were tested, with 12 testing positive for the virus. During the period of May 24–June 1, six of 18 students in a separate grade at the school — all also too young for vaccination — received positive COVID-19 test results.

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Eight additional cases were also identified among parents and siblings of students in the two grades. Among those cases, three were in people who were fully vaccinated in accordance with CDC recommendations. Of the 27 total confirmed cases, 22 people reported symptoms with the most frequently reported symptoms being fever (41%), cough (33%), headache (26%) and sore throat (26%). All of the 18 available specimens in the investigation were identified as being the delta variant, the CDC said.

Among the five infected adults, one parent (in addition to the teacher at the center of the outbreak) was unvaccinated, while the others were fully vaccinated. According to the health department investigation, the vaccinated adults and one unvaccinated adult were symptomatic with fever, chills, cough, headache and loss of smell. The investigation found that no other school staff members reported becoming ill. The CDC said no one infected in this outbreak were hospitalized.

In response to the outbreak, the CDC report said the Marin County Public Health Department held two testing events at the school as part of outbreak control. A total of 231 people were tested, including 194 of 205 students, 21 of 24 staff members and teachers, and 16 parents and siblings of students.

Following the outbreak, infected persons were isolated for 10 days after onset of symptoms (or positive test date for asymptomatic cases). All students with known exposure to an infected person were quarantined at home for 10 days following their last known contact. Unvaccinated household and community contacts were instructed to quarantine for 10 days following their last known exposure to an infected person. Officials said the two affected classrooms were closed and sanitized during May 21–30 and May 24–June 2, respectively.

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The CDC did not release information on which elementary school in Marin County was involved in the COVID outbreak.