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A new report from Ottawa Public Health into the prevalence of COVID-19 in local schools suggests that the vast majority of students and staff who tested positive for COVID-19 were initially infected outside of the school setting.

However, a decline in testing may mean more transmission went undetected.

The report looked at cases of COVID-19 in schools across Ottawa's four school boards and in private schools between Sept. 1 and Nov. 30, 2020. It found 888 confirmed cases of COVID-19 in school attendees (either students or staff) and concluded that 85 per cent of those people were infected outside of school. It also found, however, that 560 cases among in-person attendees had been at school for a portion of the time they were considered contagious.

OPH said its estimate of the in-person student population for the four boards was 124,211 students but the boards did not say how many staff were working in person during that time.

Fifty-five school outbreaks were declared between September and November, affecting 43 staff and 157 students. OPH said that total includes approximately 63 source cases who likely acquired their infection outside of school.

The largest outbreak involved 17 cases, but more than half of all 55 outbreaks began and ended with two cases.

(n.b. An outbreak in a school is defined as two or more lab-confirmed COVID-19 cases in in-person students and/or staff (or other visitors) in a school with an epidemiological link, within a 14-day period, where at least one case could have reasonably acquired their infection in the school.)

OPH notes, however, that these are only cases confirmed through testing and that testing patterns changed after September.

"Changes in testing guidelines and uptake might explain some of the decline in cases. Testing among Ottawa residents aged less than 20 years peaked in late September, just as testing was limited to close contacts and certain symptomatic individuals, and then declined in October," the report states. "In addition, the percent of tests among less than 20 years-olds that were positive increased when testing volume declined, suggesting that more cases would have been detected had there been more testing."

In late September, the province changed testing requirements to ease the burden on assessment centres and allowed children with only one symptom of COVID-19 to return to school after their symptoms cleared without a test.

The report outlined several additional possibilities for why testing levels dropped, including perceived stigma, systemic barriers, and even apathy.

"[S]ome families may choose not to have asymptomatic children tested because it does not reduce the length of their exclusion from school and they do not perceive the benefit (e.g., to identify transmission in school and to prevent further spread)," the report said. "Considering the high rate of asymptomatic infection, particularly among young people, it is possible that only a portion of cases in schools were diagnosed."

The report concluded that COVID-19 transmission within schools appeared to be low, while noting that the full burden of COVID-19 may not be known.

"The overall infection rate, measurable among students in the four publicly funded school boards, was approximately one case in every 1,000 students. Furthermore, most outbreaks were contained to a single secondary case with exposure within the school setting; 85% of cases in school acquired their infection elsewhere," the report states. "However, due to low testing rates, it is possible that some infections went undetected and that there was more spread of COVID-19 within schools than it would appear."

The report ended by saying pandemic control measures, such as wearing masks and physical distancing, are physically and mentally taxing on parents, students, and staff and that taking children out of school for two weeks to avoid transmission disrupts learning routines. It noted, however, that these measures are a net positive when compared to schools being closed.

"[T]hese efforts need to be weighed against the burden and concern for the children and youth’s physical and emotional development and well-being, as well as families’ mental health which may be affected when schools are closed," it said.

Students in Ottawa returned to in-person learning on Monday after several weeks of remote learning following the Christmas break.