Ottawa health officials don't know the source, or are still lacking crucial information, for more than a third of all COVID-19 infections in the nation's capital — and some experts say that's concerning.Ottawa Public Health (OPH) categorizes the source of COVID-19 infections under five labels: outbreak, close contact, travel, no known source and no information available.Based on the latest numbers reported Tuesday, unknown sources of infections and cases with no information available have made up more than 36 per cent of Ottawa's 5,546 cases since the start of the pandemic."That number to me was concerningly high," said Patrick Saunders-Hastings, an epidemiologist risk scientist and manager of life sciences and environmental health at Gevity Consulting. "[It] suggests that there is a weakness or shortcoming in our contact tracing and testing ability."What does 'unknown' and 'no info available' mean?This is how OPH defines both categories: * No known source means the person with a positive case was asked about risk factors and exposures, but "no source of exposure was able to be identified." * No information available means people who test positive "have not been asked about risk factors and exposures yet," and they haven't been identified as a close contact to another person with COVID-19."No known source in particular are those where there's no epidemiologic link," explained Saunders-Hastings. The no-information category in particular is "a bit of a black box," he said, because those cases haven't been traced or followed up. In early October, the city's medical officer of health Dr. Vera Etches called Ottawa's contact tracing system "nearly broken" under the current demand. Last week, OPH said it would focus contact tracing on high-risk spreaders.> Unless we control those sources, we are not going to get a handle on the COVID situation. \- Dr. Smita Pakhalé, U of O associate professor of epidemiologyOPH said in an email to CBC News that though the no-information category may appear "high at first," it's readjusted over time as diagnosed people give them more information. "This is a stressful time for those individuals, who are often feeling unwell, and it can be a difficult process that takes time," wrote a OPH spokesperson.Why do those categories matter? As of Tuesday, OPH was reporting 796 cases with unknown sources, and 1,280 cases with no information available."The higher that number is, the more cause for concern there would be," said Saunders-Hastings.In an ideal world, health officials would know the source of infection for every case — but that's not possible realistically, he said. Not knowing sources of infections could "diminish" public health's ability to respond to COVID-19, Saunders-Hastings said."They don't help us target where transmissions are occurring," he said. "They are missed opportunities to refine and tailor our response strategies."Saunders-Hastings added that the city "may no longer be able to keep up with the surge," and that might lead to further restrictions."We're currently experiencing more cases, or possible cases, than we are able to deal with."Lack of knowledge 'very dangerous'Not knowing the sources of infection is "very dangerous" for community transmission, said Dr. Smita Pakhalé, staff respirologist at The Ottawa Hospital and a University of Ottawa associate professor of epidemiology.> People still have to do their part and limit their number of contacts. \- Dr. Theresa Tam, Canada's chief public health officer"If we do not know that information, then all those people [with COVID-19] may not be self-isolating and [there] may be potential of spreading to some others," said Pakhalé. "Unless we control those sources, we are not going to get a handle on the COVID situation."WATCH | U of O prof says Ottawa's marginalized people affected disproportionately:Pakhalé also suggested there's a chance marginalized people could make up a large part of the category with no information available."A lot of people who are living in the margins of society — people who are homeless or at risk for homelessness, or racialized minorities — have been disproportionately impacted," Pakhalé said.WATCH | Dr. Theresa Tam on a third of unknown sources in Ottawa:The city's vulnerable often don't have a phone, stable housing nor equal access to information via the internet, said Pakhalé, who also leads the Bridge Engagement Centre research clinic, which works with Ottawa's marginalized communities."We don't have information about them, and ... maybe a lot of them [are] represented in that [no information available category]," she said. "And that is a very unfortunate reality of our unequal society today.""It is concerning because as we heard, the public health system['s] capacity is not limitless," said Canada's Chief Public Health Officer Dr. Theresa Tam during a news conference Tuesday, in response to a question about the sources of one-third of infections remaining unknown."So people still have to do their part and limit their number of contacts."