Up to 25% of B.C. high schools could have a COVID-19 case on day 1 of reopening, says mathematician

Up to five per cent of elementary schools and 25 per cent of high schools could have an infectious COVID-19 case coming in on day one of reopening, warns Caroline Colijn, a mathematician at Simon Fraser University.

Colijn, who is also a Canada 150 research chair in mathematics for infection, evolution and public health, has done key work plotting potential scenarios for possible arcs of the COVID-19 pandemic in B.C. Speaking with CBC's On The Coast, she said she has been crunching numbers on potential case counts in B.C. by the time schools reopen on Sept. 10.

"We came up with a few thousand cases that will be out there and that includes people we've tested and also cases we don't know about. There won't be as many in young people and children, but still, despite that, some good portion of schools, maybe five per cent, maybe 25 per cent for high schools, may have an infectious case coming in on day one," she said.

"Not all of those cases will cause outbreaks. Hopefully, some people will just get better. If it does happen, unfortunately, the size really depends on the number of people in the room, how long that contact is for, and the intensity of that contact."

Colijn said the concept of exponential growth, a pattern of data that shows sharper increases over time, is key to understanding how the pandemic grows. One aspect of that model is to look at doubling time, or the amount of time it would take for case counts to double in the province.

"When we first started thinking of this it was two or three weeks ago, we didn't even have as many cases as we have today, and if you keep that kind of idea of doubling time — what we see now is cases are doubling every 15 to 18 days, so if we see 125 today, that's 250, 500, 1,000 — it doesn't take long for it to really get out of control," she said.

"That's the direction where we're headed, if we don't stop it. I think that's the concern now. It's that kind of combination of exponential growing cases in the community but then also schools and between classes themselves. "

Need to lower community spread

During her news conference Monday, Provincial Health Officer Dr. Bonnie Henry urged B.C. residents to pull back on some of the social interactions they allowed themselves this summer.

"After many months of restrictions, we all needed to reconnect with our family, our friends this summer. We travelled, we enjoyed our summer, and we recharged," Henry said.

"As the cooler weather arrives, we all have to be ready. As we step into our offices, our workplaces, our schools, we need to take a step back from some of the social interactions that we've had this summer."

Most B.C. students will return to classes in mid-September, with students into cohorts or "learning groups" to allow for social interaction while limiting the potential for widespread transmission. Elementary and middle school learning groups will have a maximum of 60 students, while those in secondary school will have up to 120. 

Colijn said a key aspect of reopening schools without widespread transmission is to lower community spread, especially in areas like the Lower Mainland. She also emphasized the importance of schools keeping learning groups as small as possible.

"I think the idea that we're going to have to scale back on other activities to make room is a really potent one. There's a certain amount of contact that we can have. If we want to have that contact in high schools, we have to take that contact out of other places and we have to really watch for transmission in schools," she said.

"If you have to phone 60 people if someone tests positive, that's a huge challenge and a huge number of people to isolate."