Corresponding with a journalist from SW Ontario just now, made me look up the numbers. Some Ontario hot spots are those that share border with Michigan.

In infectious disease modeling, we talk about "gravity" in the spatial movement of epidemics. The gravity constant (yeah, the one from physics) describes gravity as proportional to the product of masses of two objects, divided by distance between them, squared.

something like G = k*(m1*m2)/d^2 (k being a constant)

We do that in infectious disease modeling to describe how an epidemic in one place influences an epidemic in another. m1 and m2 may be population size; d = distance between them. Or some proxy for connectedness, anyway.

We were able to use this very simple approach to predict movement of cholera in Haiti, e.g.,

https://www.annals.org/aim/fullarticle/doi/10.7326/0003-4819-154-9-201105030-00334

Others have done this for influenza: https://science.sciencemag.org/content/312/5772/447.long

(the above paper is a classic...every author in that list is a heavy hitter in infectious disease epi world). As I recall the same group also looked at spread of pandemics...

and suggested that the tight air traffic connections between NYC and LA meant that they had a lot of gravity on each other, even though physical distance was large.

At any rate...

The Michigan numbers are interesting. Michigan has 1/4 the population of Canada (10 mil. vs. ~ 40 mil.) but similar numbers of cases (30k vs 36 k), and more deaths, which may suggest they are missing more than we are (2500 dead vs 1600 or so right now).

It does seem plausible that there is gravity exerted by the Michigan epidemic on border areas in Ontario (Windsor-Essex, Lambton). I know Canadian HCW move back and forth; those are also border crossings so trucks and goods are moving.

The journalist was interested in the question of whether Michigan could be influencing outbreaks in those Canadian health units, and while I don't know for certain, my best bet would be that it is.

This reinforces the early messaging from @DrTedros: we are as strong as our weakest link.

As Ontario tries to climb down in the coming months, a key challenge is going to relate to the different epidemics, and epidemic responses, on our border.

Last point: Michigan represents another "control group" for what happened in Ontario. It is a slightly smaller population, with a similar economy, and Michigan has 5x the deaths and 3x the cases. That is not an accident. Premier Ford's action on March 14 saved many lives.