Why a 1% infection rate might not be a win for Ottawa
Consider the risk to the community, not just to yourself, epidemiologists caution
Ottawa's estimated one per cent COVID-19 infection rate creates a concerning paradox.
While it shows individuals face a low risk of getting sick here, the community as a whole remains vulnerable because the vast majority of us don't yet have the antibodies needed to fight off the disease.
In other words, the fact that so few of us have gotten sick actually heightens the risk that we will. Epidemiologists say that's why our collective action is critical at this moment to keep the virus under control.
The potential for growth is still the same as it was back in March.- Dr. Doug Manuel, The Ottawa Hospital
Raywat Deonandan, an epidemiologist at the University of Ottawa, says on balance, the city's one per cent rate infection rate is ultimately a good thing, even though it means we're nowhere near levels required to achieve herd immunity — when most of a population is immune to an infectious disease, providing indirect protection to those who are not.
"If it was really high it would be good, if it was really low it would be good. It's the in-between stuff that's not great," he said.
Despite the current risk of infection being so low — when you walk out the door, there's only a slight chance of encountering an infected person because so few of us have contracted COVID-19 — there's still a strong community responsibility to keep up physical distancing, Deonandan said.
"When we scale this up by the millions, the risk goes up for the community," he said.
It's a bit like voting. One vote may not feel monumental, but the collective action can change the course of a whole nation. Wearing a mask or staying two metres apart may feel like a small step, but if most of us do it, the virus retreats.
This point in the pandemic, Deonandan said, calls for "a shift in the way we have to think about our behaviours."
Transmission tracking by Ottawa Public Health (OPH) shows community outbreaks were responsible for a handful of new cases in April, but have since fallen to zero.
"Sporadic spread," the technical term for COVID-19 transmission that isn't connected to a known outbreak or cluster of cases, is reported to be responsible for 10 or fewer cases a day since the start of May.
Potential for spread remains high
Even by Canada standards, Ottawa's one per cent infection rate is low. Dr. Doug Manuel, an epidemiologist and senior scientist with The Ottawa Hospital, says Canada as a whole is estimated to have had a four or five per cent infection rate.
New York state and northern Italy put their rate at roughly 15 per cent, he said.
Manuel, who helped design the modelling that led to the one per cent estimate, said it's based on how many people have tested positive, but also how many people have died or been hospitalized with COVID-19.
"I think of it as sort of a pyramid, and toward the top of the pyramid are hospitalizations and deaths," he said. "When we work backwards from that we have even less than one per cent."
Antibody testing wasn't used to inform the one per cent estimate, Manuel said, but conversations have started about how to use the data that can tell whether someone has previously had COVID-19. OPH says it's waiting on guidance from both the province and federal government on how to implement antibody testing.
Once antibody testing is underway, it should give health officials an even more reliable picture of COVID-19's spread.
Vulnerable to 2nd peak
Manuel remains confident though that while low, one per cent of Ottawa's population — around 10,000 people — is close to the actual number of infections.
In Ottawa's long-term care homes, where COVID-19 has killed scores of older adults, nearly all residents have now been tested for COVID-19 and only about eight per cent of them tested positive, he said.
"Our hardest-hit area is at eight per cent," Manuel said. "If we had eight per cent in our community, our hospitals would be really overflowing."
Were it 20 per cent, along with jammed hospitals, about 3,500 people would be dead, he said.
Like Deonandan, Manuel believes the city has done well against COVID-19, but he also fears such a low infection rate leaves Ottawa vulnerable to a second peak as individuals become more relaxed about the pandemic. Low risk for individuals does not mean the entire community is safe.
"We can socialize, but we have to watch our contact," he said. "The potential for growth is still the same as it was back in March."
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