It has been two weeks since B.C.'s September 10 school reopenings, a time period generally considered to be the outside limit for the onset of symptomatic COVIID-19 infection.
There have been no officially reported cases of school "outbreaks" or "clusters" of coronavirus infections in the province, but there have been, as of today (Septenber 24), 31 "exposures" in B.C. schools.
In addition, news outlets have reported four exposure incidents at Vancouver Coastal Health (VCH) schools that do not appear on the regional authority's website. (VCH issued a joint statement with B.C. provincial health officer Dr. Bonnie Henry to respond to concerns about its approach, and Henry spoke further about the issue at the B.C. COVID-19 update on September 24.)
Fraser Health's 20 exposures leads B.C.'s five health regions, by far. VCH's five exposures rank second, with Northern Health and Interior Health's three apiece coming next, and Island Health's zero tally claiming last spot.
(In contrast, more than 550 schools in Ontario and Quebec had reported at least one case of COVID-19 by September 23, with Quebec accounting for more than 70 percent of that total.)
Surry School District 36—B.C.'s largest, with more than 70,000 students—represented 12 of Fraser Health's 20 official exposures, 60 percent of the region's school cases and about 40 percent of the provincial total. (Fraser Health covers 20 communities from Burnaby to Hope.)
The population of Surrey—according to the 2016 census, the latest statistics available—is 517,887, while Vancouver proper logged in at 631,486.
A school "exposure" is defined by B.C. health authorities as "a single person with lab-confirmed COVID-19 infection who attended school during their nfectious period".
A "cluster" is described to be "two or more individuals with lab-confirmed COVID-19 infection who attended school during their infectious period", while the term "outbreak" is used for "multiple individuals with lab-confirmed COVID-19 infection when transmission is likely widespread within the school setting."
In all the above cases, public-health officials investigate and initiate contact tracing.
Almost half of the reported 31 B.C. school exposures, 14, have taken place in secondary schools, and 11 of those are from Fraser Health, with Surrey accounting for nine of that total. That Surrey number represents almost 80 percent of the region's exposures in high schools, and about 65 percent of the provincial high-school total.
According to mathematical modelling published in the Straight that was done by an SFU epidemiologist and mathematician, Caroline Colijn, and three colleagues before the September 10 school reopenings, secondary schools in the VCH and Fraser Health regions could experience "dozens to hundreds" of COVID-19 "outbreaks".
Regarding their paper then under peer review, they wrote in an article on August 14: "The chance that a school with 1500 students has at least one entering who is infectious is 20–30%. That’s a large number of high schools in the VCH and FH region that are at serious risk of an outbreak, right away."
The article predicted that "a single asymptomatic student could infect between 10 and 25 other students in a single week. Then, before they know they are ill, those new cases could infect many other students, family members, teachers and other contacts. This could lead to outbreaks in the dozens to hundreds."