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Classroom interaction between a teacher and students could lead to Covid-19 transmission, according to a small study by US Centers for Disease Control and Prevention researchers published Monday in Emerging Infectious Diseases.

The report details a case where a teacher infected with Covid-19 may have infected students after returning to the classroom while still experiencing symptoms. The teacher first had symptoms, including a headache, sore throat and fatigue, while traveling in Europe in late February. The teacher returned to school while still experiencing symptoms and taught 16 classes of 30 or less students. After the teacher’s Covid-19 test came back positive on March 1, all students were told to quarantine at home. Once the quarantine was over, 21 students from the teacher’s classes took part in the research. The students had a median age of 17.

Ten of the teacher’s classes were described as “interactive” classes, in which the teacher walked around and spoke directly with students. Out of five students who had been in “interactive” classes, two had antibody test results that suggested they had been infected. One of those students experienced symptoms for nine days, include muscle aches, runny nose and cough and the other student had no symptoms. Two other students from the five in the “interactive” classes had no serological evidence that suggested they had been infected in the past, but they had reported mild symptoms – one had fever and headache that lasted one day, and another had a runny nose that lasted one day.

Of the 16 students who participated in the non-interactive classes – where the teacher sat mostly in one location and had limited close interaction with students — seven reported symptoms, including sore throat, headache, runny nose and muscle pain. But antibody tests found no evidence of a previous Covid-19 infection. 

“Widespread school closures have mostly eliminated the risk for classroom transmission,” the report says. “However, these results suggest that classroom interaction between an infected teacher and students might result in virus transmission.” 

The authors note there are limitations to the research, including a low number of participants and the possibility that students may have been exposed to Covid-19 somewhere other than the classroom. Also, blood samples may have been collected too soon for students to have developed antibodies to the coronavirus.