High number of COVID-19 cases in Sask. with 'unknown' source not surprising, experts say

·4 min read

The province says household exposure is the top source of COVID-19 transmission in Saskatchewan, in cases where the exposure source is known.

However, exposures whose source is listed as "pending" and "unknown" far outweigh all known exposure types combined.

That information comes from a table presented by Chief Medical Health Officer Dr. Saqib Shahab on Monday.

According to the provincial data, since the beginning of the pandemic in March, a total 6,086 recorded cases, as of Dec. 13, fell into the "pending" or "unknown" transmission source categories.

In comparison, cases with a known exposure type — such as exposure to the illness in a household, a workplace, at a mass gathering or through travel — add up to 3,880 known exposures.

Province of Saskatchewan
Province of Saskatchewan

The Saskatchewan Health Authority noted those totals do not match the number of cases reported in provincial COVID-19 updates, because the daily update numbers are based on the date of the laboratory test report. The province's recent exposure table, however, is based on the date symptoms began.

Dr. Cory Neudorf, a public health physician and professor of community health and epidemiology at the University of Saskatchewan, says he is not surprised that cases with an unknown exposure source outnumber those where the source is confirmed.

"This is what we were predicting ... because once you reopen up activities and businesses, you're just increasing the opportunity for the virus to spread, especially among those who are asymptomatic," he said.

Saskatchewan has ramped up its testing, Neudorf said, including among those who aren't showing any symptoms.

"[That] means there's more people who have turned out to be positive, but they don't know when it is they picked up the infection because they haven't shown any symptoms," he said.

"And so how do you do contact tracing with those individuals? They don't know when they might have been spreading the disease and they probably didn't change their behaviour, because they weren't symptomatic."

Those people might not even get tested, he said, since they aren't showing any symptoms.

Saskatoon Health Region
Saskatoon Health Region

Between Dec. 6 and Dec. 13, the province logged 94 exposures that happened within the household. There have been 1,195 known household exposures since the beginning of the pandemic.

"We know that the bulk of this is happening in at least occasional communities spread and then we're seeing secondary passing off in household settings," Neudorf said.

"We're not seeing as many of the big superspreader outbreaks because those things aren't allowed right now. But there is this ongoing, more casual contact happening in whatever place people are congregating. So that must be including businesses and workplaces, break rooms and casual contact."

Neighbours to the east

Manitoba is also struggling to keep up with contact tracing.

"In every province, the more cases there are and the more community spread there is, the higher the proportion is of cases of unknown contacts," said Cynthia Carr, an epidemiologist who is the founder of EPI Research Inc. in Winnipeg.

"And that's a sign of community spread."

Carr says that's why it is important for Canadians to use the COVID alert app, a free exposure alert tool.

It's difficult for provinces to keep up with contact tracing interviews, she said.

"Remember that for any case that's identified today, there has to be … the positive test result, the interview with the case, the identification of the contact, the followup with the contact, and then time for those contacts will become cases," said Carr.

"So even if contact tracing is up to date, which is very hard with the number of cases, there's still that lag in trying to figure out how many of those contacts convert to cases."

Carr says that does not mean the cases listed as having an "unknown" exposure source will stay that way.

"There's all kinds of later epidemiological research that could be done to link [the cases]."

Adam Hunter/CBC
Adam Hunter/CBC

Saskatchewan's chief medical health officer says the province continues to struggle with contact tracing.

"Whenever you don't have an outbreak like two or more cases linked to an assessment, you can't really say where transmission happened," said Shahab.

"But we do know that for the most part, it seems that … if you [are] staying home all the time and not going out, even for shopping, your risk is very low."

Shahab says the average number of close contacts for each person diagnosed with COVID-19 has dropped from eight to five, which should help in contact tracing efforts.

CBC News Graphics
CBC News Graphics

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