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I didn’t expect this.

Yes, I thought we’d see a rise in Utah coronavirus cases after Labor Day and once schools opened. It was clear that was going to happen. But the acceleration and sheer size of the spike we’ve seen, going from 400-ish cases each day to more than 1,000 in the course of a week... well, it blows my socks off.

In particular, much of the rest of the United States is inviting kids back to school, too. Everyone had Labor Day, a federal holiday. And yet, Utah’s coronavirus growth is huge, where the nationwide case numbers have stayed pretty flat.

Some also wondered if the spike was due to bunched testing. After all, Labor Day and the windstorm slowed testing for a few days, almost a week. But looking at the numbers doesn’t support that claim — testing is only up marginally, but positivity rates are up to their highest levels.

What’s going on? The biggest part of Utah’s spike is tied to a specific place and demographic.

Utah County’s woes

Utah County isn’t solely responsible for the rise in cases — other areas of the state are increasing, too. But Utah County is seeing rises on a per-capita basis that this state has never seen before.

Here’s a graph of Utah County’s daily cases per 100,000 residents compared to the rest of the state. It’s pretty jarring:

Utah County’s case averages stayed pretty similar to the rest of the state’s during most of the pandemic. But, in August, cases decreased less there than elsewhere. And since September began ... woo, boy, cases have exploded.

Consider that 2,135 cases of the coronavirus have been found in Utah County over the past week. With an estimated population of 636,235, that means about one in 298 people have tested positive for the virus just in the past seven days. How does that compare to other more famous hot spots?

You’re probably aware of the May outbreak in the Navajo Nation in southeastern Utah, where San Juan County was Utah’s hottest spot, garnering national attention as well. In its highest seven-day period, that area had 98 cases out of a county population of 123,958, or one in 1,264 people. Utah County has been about four times as dangerous.

You’re definitely aware of New York City’s coronavirus outbreak in March and April. In its worst seven-day period, the city had 37,107 cases, out of a population of 8.4 million. That means 1 in every 226 people caught the virus that week — not too far off of Utah County’s numbers.

You can do this for the most famous (infamous?) coronavirus hot spots.

Testing has changed since earlier in the pandemic, and we’re probably catching more cases. And yet, it is still very troubling to see Utah County up there with the worst outbreaks we’ve seen.

Young folks getting it first

But sirens aren’t blaring throughout the day in Utah County like they were in New York City and Italy at their coronavirus peaks. Why?

First, a look at the hospitalization numbers, which haven’t seen recent growth.

Deaths, too, have stayed relatively flat. We’ve only seen 10 coronavirus deaths this week in Utah, with only two of those coming from Utah County.

Why is that? Well, this outbreak is exceptionally concentrated among young people. Here’s how the estimated seven-day mean (average) and median (middle point) have changed in Utah’s daily case counts over the course of the pandemic:

The Utah Department of Health releases only an age range for each case, not a specific age. Still, by looking at how many cases are in each age range, we can estimate an average and a middle point for each day’s cases. And recently, those estimates have absolutely cratered: We’re now estimating that the median coronavirus patient in Utah is 26 years old.

Note: We can’t say that all of these kids are from Utah County. The state doesn’t release age numbers by county, so we don’t know if these kids are disproportionately from Utah County or from all over the state.

Regardless, young people don’t get extremely sick very often: only about 1.5% of people with the virus in the 15-24 age group have been hospitalized so far in Utah, and 3.7% of 25-44 have been hospitalized. Compare that to elderly populations: 22.1% of cases in those 65-84 are admitted to the hospital at some point.

If these cases stayed in the young population, I’d be less concerned. But young people mingle with old people. Kids hang out with their parents, who then hang out with their parents. College students are taught by older adults. At-risk elderly are treated and cared for by younger health care workers. Everyone goes to stores.

It is reasonable to expect that we’ll see a rise in cases among Utah’s older populations and we will see deaths. It’s going to take a couple of weeks for the cases to fully transmit from young to old, and then a further few weeks for sicknesses to turn to death, but it will happen.

I don’t think we’re going to see New York City-level death, though. Quite frankly, we’re just a lot better at treating the coronavirus than we were in those early days — it turns out that putting a ventilator on everyone with low blood oxygen levels is actually pretty dangerous. We’ve found some treatment options that help some patients, but not all.

Still, Utah’s spike is concerning. Our transmission rate is now well above one, which makes sense after we doubled cases in the past week.

To control the pandemic and prevent hospitals from being overrun, we have to slow the spread. We have to enact interventions that will lower our transmission rate — that means masks, restrictions on schools and large gatherings.

This is bad. It’s time to start acting like it.

Andy Larsen is a data columnist who is focusing on the coronavirus. He is also one of The Salt Lake Tribune’s Utah Jazz beat writers. You can reach him at alarsen@sltrib.com.