Ontario can’t identify origins of most coronavirus cases
◼ Source of exposure unaccounted for in two-thirds of reported incidents ◼Province’s difficulty in tracing cases stands in contrast with B.C., Alberta
As Ontario begins to reopen and works to contain the spread of COVID-19, one aspect of the fight has proved troublingly evasive.
On any given day in the past month, the province hasn’t been able to account for the source of exposure for more than two-thirds of reported cases of the virus. This is because the cases are due to community spread — where the source has been determined to be “unknown” — or they are still being investigated by local public health units.
Ontario’s difficulty in tracking down this information stands in sharp contrast to other provinces, namely British Columbia and Alberta, which have been able to follow the virus better, contain it and ease lockdowns faster.
“When you know the source, then you know where the hotspot is,” said Colin Furness, an infection control epidemiologist at the University of Toronto. “When you don’t, you’re left guessing.
“Overall, Ontario’s performance has not been good, but it’s variable by region. In some areas, there have been very few cases and contact tracing has been very well done. In other regions, they’ve collected a lot of data but they haven’t reported it. And in other places, they’ve just been overwhelmed.”
Every day, Public Health Ontario publishes a summary that breaks down new and cumulative cases of COVID-19 into categories such as gender distribution, hospitalizations, outbreaks in longterm-care homes, and deaths. The information is pulled from the province’s integrated Public Health Information System (iPHIS), which relies on local public health units to enter case data by 4 p.m. the previous day.
The summary also reports the number of confirmed cases by exposure type in four classifications: travel, close
contact of a confirmed case, community transmission and information pending.
Over the last month, the “travel” and “close contact” exposure types — for which Public Health Ontario knows whether the virus was acquired through travel or exposure with people who had the disease — have together averaged about 30 per cent of the province’s total cumulative cases.
The remaining two classifications, “community transmission” and “information pending,” are problematic in that officials don’t know on any given day how people in these categories were exposed to the virus. Since April 21, these two categories have accounted for more than two-thirds of the province’s total confirmed cases on a daily basis.
For example, on May 20, there were a total of 24,187 cumulative confirmed cases of COVID-19 reported in Ontario, according to the summary. Of those, 1,557 were the result of exposure to the virus due to travel, 6,030 due to close contact with a confirmed case, 8,412 were the result of community transmission (i.e. we don’t know where people got it and neither do they), and 8,188 were “information pending.”
In other words, cases for which the source of the virus is unclear and for which we have incomplete information together made up 68.6 per cent of the province’s total confirmed cases that day.
The definition of community transmission varies among some regions, but Dr. David Williams, Ontario’s chief medical officer of health, told the Star the province classifies it as cases where “we can’t be sure where they got it from — and (the patient is) not sure.”
He said the province is trying to reduce the “community transmission” and “information pending” cases, noting that cases of the former are particularly worrisome.
“We don’t know if they got it from a contact. We don’t know if they got it from travel. We don’t know if they got it from an outbreak. They’re just out there.”
Williams said the goal is to ultimately find out how people got the virus.
“It is difficult to determine exactly where someone acquired their infection because the incubation for this virus is approximately 14 days,” said Dr. Vinita Dubey, Toronto’s associate medical officer of health. “It is sometimes not possible to determine exactly what was the source for the person’s infection.”
A breakdown provided Wednesday in an email by Toronto Public Health listed as unknown or under investigation the source of transmission in 20 per cent of their 8,866 cases — 1,773 people. It lists as community transmission only 10 per cent of the total cases — 886. (Other categories are: 31 per cent close contact; 27 per cent related to institutions; eight per cent related to health-care settings; four per cent related to travel.)
In Ontario, the number of cases attributed to travel have not increased much since the early days of COVID-19 contact tracing. The other three categories have risen steadily, and relatively evenly.
When contact tracers assign a case to a category they make a judgment, and that really comes into play with “close contact” and “community transmission.” Close contact is relatively easy to assign — say a husband is a confirmed case and his wife also tests positive. Or an infected resident at a long-term-care home passes the virus to a personal support worker.
Furness said community spread is “a very long process of investigation simply to come to the conclusion that there’s no connection to be found,” Furness said. “When you have a lot of community spread, it’s a double whammy. You get a lot of cases and you’re going to spend a ton of time on each case to come to that conclusion that you’ll never know where it came from.”
Speaking to the media Wednesday, Williams said “information pending” cases would shift into other categories as investigations continue. “Ideally, I’d like to see that number dropping down low in the long term,” as public health officials trace cases to an outbreak, close contact or other epidemiological link.
British Columbia and Alberta, two provinces hailed by experts for their swift and organized responses to the COVID-19 pandemic, seemed to have fared better in tracking down sources of exposure to the virus.
As of Wednesday, British Columbia has had 454 local cases with an unknown source, out of its 2,465 total cases of the virus, or 18 per cent. In addition, 92 cases, or four per cent of the total, have information pending or missing.
Of Alberta’s 6,735 total cases, 624 are currently classified as “unknown exposure,” or nine per cent. Meanwhile, the province has linked 3,752 cases directly to an outbreak of the virus as of Wednesday, like those at meat packing plants or longterm-care facilities.
Alberta considers cases to be “unknown exposure” if a link to an outbreak or confirmed case can’t be found, a representative for Alberta Health Services (AHS) told the Star. AHS’s 270-person contact tracing team begins investigating cases within 12 to 24 hours of receiving lab test results, and “contact investigation is also initiated within that time frame,” the provincial health service said.
Furness attributes B.C.’s comparably better numbers to good data collection and the fact that it didn’t have as much community spread as Ontario.
“Do they have better record keeping? Yes, because no one could be worse than Ontario,” he said, noting that B.C. has seen only one-tenth the number of cases Ontario has. “So they’re not overwhelmed in the same way we are.”
Ontario’s health ministry has said that case and contact management by local public health units will inform the government’s decision-making as the economy reopens. “To do so, public health units are being asked to contact 90 per cent of confirmed positive cases within 24 hours of being notified of the case,” said Hayley Chazan, a spokesperson for
Health Minister Christine Elliott.
Chazan said 94 per cent of public health units are currently meeting this target. She said the province is currently exploring using apps to help increase the volume of contact tracing.
Contact tracing “can be grunt work,” said Raywat Deonandan, an epidemiologist with the University of Ottawa. “Tracers are likely investigating a case’s recent history and likely exposures. There is also likely a delay in the information pipeline between the investigators’ appraisals and the data collection centre,” he said.
According to Toronto Public Health’s Dubey, the city’s case managers and contact tracers number 500, with more being trained. The staffers are now supported by 45 volunteers, said Dubey.
“We therefore continue to investigate many COVID-19 cases in our city,” said Dubey. “We identify those interactions where the individual had close and prolonged contact with others and follow up directly with those people. This often includes individuals in households, workplaces and social events.”
While Deonandan said it’s tempting to point fingers at public health leaders, Ontario is “also a more populous province with complicated demographics and several large cities.”
British Columbia and Alberta have a third of Ontario’s population, he noted. “However, both B.C. and Alberta expanded testing criteria earlier, and are more transparent with their data processes; that says something about leadership.”
Ashleigh Tuite, an associate professor of epidemiology with the Dalla Lana School of Public Health, said that based on public data, “there’s nothing that’s strongly convincing or reassuring that we have (COVID) under control.”
Speaking broadly about whether Ontario should be reopening given current trends, Tuite said it’s important to look region by region.
“There are parts of the province where there really hasn’t been a lot of activity for a fairly long period of time,” which differs from denser areas such as the GTA, Tuite explained. “But I think having a really clear understanding of where we are, where transmission is happening and what the plan is moving forward is critical.”
“B.C. and Alberta expanded testing criteria earlier, and are more transparent with their data processes.” RAYWAT DEONANDAN EPIDEMIOLOGIST, UNIVERSITY OF OTTAWA