Hamilton can expect to see its maximum number of COVID-19 cases by early to mid-May, with the next couple of weeks likely to bring a significant surge in both cases and hospitalizations.

"Right now with the number of hospitalizations, ICU ventilators, one can say there's essentially been no untoward impact," said Dr. Doug Sider, a Hamilton public health physician and epidemiologist.

"But we're going to start seeing more hospitalizations, more critical care ventilated patients almost certainly," said Sider. "I just can't tell you what the numbers will be."

No one can. In fact, no one can say with any certainty how many cases we have in Hamilton now.

What we're measuring and what's important for us to know aren't the same thing, said Jonathan Dushoff, a McMaster University theoretical biologist who studies infectious diseases.

"A case of what?" Dushoff asks. "Is it a case of infection, meaning did you get it at all but maybe with no symptoms? And we don't usually call that a case.

"Is it a case of illness? Is it a case of clinical COVID disease? Is it a case of hospitalization? Is it a case of a severe outcome?

"All those are different things but we call them all cases," said Dushoff. "None of them are what we're counting. What we're counting are laboratory confirmations."

In Ontario, we don't have the ability to test everyone for COVID-19 so the testing that does happen is haphazard. There have been nearly 50,000 tests carried out in Ontario as of Sunday morning, or about three tests for every 1,000 people.

On top of that, there's a been a significant backlog in test analyses.

Some infected people have no symptoms and wouldn't know they're infected.

Some infected people have mild symptoms. They aren't tested and are told to simply stay home in isolation.

Even if we could test everyone, a test is only a marker of a point in time. A person could test negative last week and positive this week.

So the case numbers being reported in the media each day are cases confirmed from tests, not the true number of people infected with the virus.

"What we want to know is what proportion of those infections are going to lead to severe outcomes," said Dushoff.

"But we're not measuring infections," he said. "We're measuring laboratory confirmations which are this strange sample of infections."

The test results' backlog has likely kept Hamilton's numbers artificially low, Sider said.

The province is also ramping up the number of tests that can be carried each day, hoping to hit 19,000 tests a day by mid-April.

"What could well happen is the numbers over the next few weeks could go up quite a bit," Sider said.

"What this might mean is not that there's more and more infection, it could mean the underdetection of the virus is decreasing and so we're getting better at detecting it."

The models for Hamilton are based on a 14 per cent detection rate. Put another way, for every one detected case, it's assumed another six go undetected.

So the 70 cases reported Sunday for Hamilton would really represent nearly 500 if the undetected cases are added in.

But what if the detection rate is much lower, say one in 20? Then there would be 1,400 cases in Hamilton.

"It's likely that the underdetection is fairly significant," said Sider. "I don't know there's any way that we can develop really precise estimates of the degree of underdetection."

There's a long list of challenges — some global, some local — that hamper efforts to predict the scope and severity of COVID-19 spread in Hamilton, including:

• It's a new strain of virus with no natural immunity in the population.

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"We assume we have no population immunity," Sider adds. "Maybe it's not quite that bad. Maybe there's a bit of cross-protection."

• It's more infectious and deadlier than normal flu-like viruses.

• Only a fraction of the population has been, or is likely to be, tested and there's a backlog in getting results from tests already carried out.

• We don't have an accurate count of the people already infected, particularly those who don't show symptoms.

"What if one in seven or one in eight is asymptomatic but infectious," said Sider. "What if it's a service worker or someone working in a hospital?

"What the infectious disease models can't really handle is if there's a lot of transmission from asymptomatic individuals."

• There's no vaccine or treatment.

"The way COVID gets under control — and it looks like the only way it gets under control — is if a big proportion of us are immune," said Dushoff.

"There are two ways to get immune — one is to have an infection, which is dangerous and we don't know super well how dangerous, and the other is to have a vaccine."

• Older people and those with compromised immune systems are more vulnerable and Hamilton has a larger than average proportion of seniors. Nearly 100,000 people in Hamilton are ages 65 and older.

"The objective from day one has really focused on how can we continue to maintain an accessible functional health-care system?" said Sider. "How can we do our best to reduce the impact on hospitalization, use of critical care beds, ICUs, ventilators and ultimately deaths due to COVID-19?"

The next couple of weeks will show if the social distancing measures are paying off, Sider added.

"I hope what we'll see is that we learned from the emerging experience of others," he said. "We jumped on this, we instituted fairly stringent, fairly widespread control measures.

"Hopefully we're not going to be an Italy or a Spain or an Iran or a China."

sbuist@thespec.com

905-526-3226