It's Saturday, September 19th. It's a couple of hours before today's Utah case numbers come out.
How do we know if the recent uptick is continuing to get worse? Here's a number for you: 634 new cases.
That's a lot less than yesterday's 1,089 new cases.
How could a big decline from yesterday be a bad thing?
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Here's how. The case count data is seasonal.
Seasonal is the word used in economics (and other fields) for data measurements that have a repetitive pattern in them. For example, employment is higher every December due to retail firms hiring more workers for holiday shopping season. Seasonality is a generic name: it doesn't always have to be related to seasons. What's important is that the pattern occurs regularly.
Case data in epidemiology is usually seasonal too. Importantly in this case, there are weekly seasonals in case data for most diseases. The reason is simple: the weekend. Doctor's offices are open less. Labs have fewer people working. Sick people who might have been out and around a bit and able to pop into a testing center on a weekday decide to hunker down at home for a couple of days, and so on.
With positive test results, they're usually higher early in the week (as the labs catch up from the weekend), and lower on the weekend when only the most urgent tests are completed.
A particularly simple way to deal with this is to look at the growth rate between the same day each week.
This is really bad folks. That's a multiple that's plotted. When it's above one and level, we'll get more cases on a day than we got the same day last week. An outbreak, could even show up in something like this: if it bumped up to a new level, and then stayed horizontal (say, we'd been getting 5% more cases each week, and suddenly we start getting 6% more).
But this multiple hasn't even leveled out yet!
Do the math. Yesterday's total of 61,775 cases was about 1.09 times larger than the previous Friday's 56,693 cases. So, if the outbreak is the same today and is was yesterday, we should have 1.09 times as many as last Saturday's 57,275. That's 62,409: an increase of 634 from yesterday.
Anything more and that upward sloping line has taken another step upwards.
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Here's some speculation on where this outbreak came from (credit to MJ). The uptick starts a week ago Thursday. That's 3 days after Labor Day.
The incubation period for COVID-19 is about 4-5 days. All of this is consistent with students away at college mixing with family back home over the holiday weekend. Given that the outbreak is mostly in the population center Salt Lake and Utah counties, and has skipped over some other counties with big student populations (Cache and Weber), I suspect the direction was from the colleges into the Salt Lake metropolitan area.
Here’s a bit more about my college to city hypothesis.
The data from this comes from the state’s dashboard, but you have to go down to the heading “Cases by Local Health Department” to see the graphs showing that it is mostly a Salt Lake and Utah county event. Just over half the state’s population lives in those two counties.
The official Salt Lake City metropolitan area is quite a bit larger, but when most people think of Salt Lake, or the Wasatch Front, they’re thinking of Utah, Salt Lake, Davis, and Weber counties, with maybe Summit thrown in. Really … Box Elder, Juab, Morgan, Tooele, and Wasatch counties are not “Salt Lake” — and Cache definitely is not — no matter what the Census Bureau says the CMSA is.
Two of the big universities in the state — the U, and BYU — are in this area. Weber State is too. And Utah is a state characterized by fewer students from other states, and few students leaving to go to schools in other states. There are some other universities in Utah, but they mostly draw from other parts of the state (and also SUU has gone to a quarter-like calendar, and only began the session just as the uptick in the statewide numbers started).
Also, it’s a given that for many college students this year, living on or near campus is less likely.
Now, follow the logic:
- BYU draws from the furthest distances, but most of those students wouldn’t go home to Utah would they? And if you are worried about living on or near campus, it’s an easy commute from Utah County, and a do-able one from Salt Lake County.
- The U is more of a commuter school already, than most flagship schools. And again, an easy commute from within Salt Lake County, and a doable one from Utah County.
- Weber State does not have as many students, and is much more of a commuter school. But it would be a tough commute to and from Utah County and the southern parts of Salt Lake County, so my guess is that a lot of those students choose to live in Ogden to get away from their parents.
- And then there’s Utah State: comparable in size to BYU and the U, and close to impossible to do as a commute from Salt Lake County or Utah County. And not much of an uptick in its Cache County.
So here’s what I think. It’s tough to imagine reasons why an outbreak would flare up after Labor Day in just Salt Lake County and Utah County. That is, unless there was an influx of people from outside into both of those counties. Some people might go from Weber and Cache counties to school in Salt Lake and Utah counties, but the numbers must be much larger going the other way.
And I think they did the easy 2-ish hour drive back home for the long weekend, bringing their admittedly low incidence levels into thousands of otherwise COVID-insulated homes in Salt Lake County and Utah County.
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