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COVID-19 data: What is the risk of coronavirus exposure in Utah ahead of Thanksgiving?

By Carter Williams, KSL.com | Posted - Nov. 23, 2020 at 3:04 p.m.



SALT LAKE CITY — Thanksgiving will likely look different this year for many Utahns.

With COVID-19 still a concern many months after it reached the U.S., local and national public health experts and leaders have pleaded over the past few weeks to ditch large family gatherings for the holiday. They've advised people to hold small gatherings — mostly within one household if possible — and communicate with extended family electronically.

It all comes down to the risk that COVID-19 will spread more than it already has. In Utah, the biggest concern is that Thanksgiving gatherings will lead to more cases, which will lead to more hospitalizations and deaths for a health care system already close to reaching intensive care unit capacity.

"You increase the risk when you bring people into your home — they're from outside the four walls of your house. That's just the harsh reality of what the data shows us," said Gov. Gary Herbert, during a press briefing Monday morning.

"Bringing people from outside your walls is going to increase the risk of somebody who will share with you the COVID-19 virus," he added.

Beginning Tuesday, the Utah Department of Health's transmission index will return to what it was prior to Herbert's emergency order earlier this month. Twenty-six of the state's 29 counties will be placed in its "high" transmission category, while Daggett, Piute and Rich counties will move back to low transmission.

That's not to say there is no risk for COVID-19 spread, even in those counties. Every county has its own risk of spreading COVID-19, especially with gatherings expected this week.

Calculating risk

One person with COVID-19 can pose concerns for others around them, and it's possible they don't even know they have the disease. The virus can spread asymptomatically and through people with mild symptoms.

FiveThirtyEight compiled an analysis of what happens when one person with the virus attends a small gathering with 10 people; it shows it's likely that one person will infect others around them. In a scenario involving 25 different households with one person at the table with COVID-19, it projected 25 cases jumping to 80 — with the 55 newly-infected people going around their community and possibly spreading it to more people at work, school, church or while they shop.

So what is the possibility of having someone showing up to the table with COVID-19 this week? Researchers at Georgia Tech launched the "COVID-19 Event Risk Assessment Planning Tool" earlier this year. It offers a look at COVID-19 risk if you plan on attending an event from as low as 10 people to as many as 5,000 people in any county in the U.S., as well as regions in Europe.

Per the website, the percentage is not exactly how likely you are to get the coronavirus at a gathering; rather, it's what is the estimated likelihood at least one person with COVID-19 would be at a gathering based on county population and COVID-19 case count data.

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Researchers also took into account the possibility that cases are underreported, which can happen if someone is asymptomatic or has a mild case that they don't seek testing for. The data tacks on an ascertainment bias of five or 10 times the reported cases to account for this possibility. An individual on the website can view data through either of these bias options toggled. The 10 ascertainment bias yields much higher risk.

Dr. Angela Dunn, epidemiologist for the Utah Department of Health, explained it's a tool used by many health experts to look at the risk in their communities. But it's also something someone planning to head out to an event can use to calculate their own risk.

"It will really help you understand, hopefully, that risk of someone at that event," she said.

Now let's see what the map tells us about COVID-19 in Utah heading into Thanksgiving.

A look at Utah's risk

Georgia Tech's model shifts days. For reference, KSL.com collected data for this story from the Georgia Tech model on Monday morning.

Let's start with 10 people at a gathering. Per the COVID-19 Event Risk Assessment Planning Tool, the risk of at least one person attending a gathering that small mostly ranges from 20% to 50% in Utah when you consider an ascertainment bias of five.

The lowest risk is Daggett County, which holds a probability of less than 1%. The state's least-populated county holds a risk of less than 1% in the model in every scenario, which isn't the case with every area in the state.

The highest risk, per the model, is Garfield County in southern Utah. It lists a 50% chance that an event with 10 people at it will bring at least one person with COVID-19 with an ascertainment bias of five; it jumps to 77% if you adjust the ascertainment bias to 10.

Here's what the probability was for every Utah county with a gathering of 10 people with an ascertainment bias of five:

  • Beaver: 20%
  • Box Elder: 34%
  • Cache: 44%
  • Carbon: 38%
  • Daggett: Less than 1%
  • Davis: 35%
  • Duchesne: 29%
  • Emery: 27%
  • Garfield: 50%
  • Grand: 34%
  • Iron: 26%
  • Juab: 24%
  • Kane: 18%
  • Millard: 19%
  • Morgan: 38%
  • Piute: 22%
  • Rich: 18%
  • Salt Lake: 41%
  • San Juan: 24%
  • Sanpete: 30%
  • Sevier: 48%
  • Summit: 29%
  • Tooele: 32%
  • Uintah: 23%
  • Utah: 44%
  • Wasatch: 41%
  • Washington: 39%
  • Wayne: 21%
  • Weber: 34%

When adjusted with an ascertainment bias of 10, the percentages with a group of 10 people nearly double in almost every location.

To no surprise, the analysis compiled by Georgia Tech shows the probability of exposure increases with the number of people in attendance. If you add another 10 people, the probabilities in most counties are near or exceed 50% even with an ascertainment level of five. The probability in highly populated Wasatch Front counties like Davis, Salt Lake, Utah and Weber all range from 57% to 69% with a group of 20 people in that scenario.

Move the figure up to 50 people, and the probability that at least one guest has COVID-19 exceeds 60% in all counties except Daggett County, per the model data. By 100 people, the likelihood exceeds 90% in 25 of 29 counties.

This is why health experts have warned against large gatherings for the holiday. There will be some possibility of spreading the virus with different households meeting, but the likelihood of spreading COVID-19 only increases with every additional household added.

Dunn said this risk led her to adjust her normal holiday traditions. She plans to have a Thanksgiving meal with only her immediate family instead of inviting friends over from around the area. It's a sacrifice she hopes others will make given what's at stake.

"It's so essential that we take this opportunity in the last several months before a vaccine becomes available to not continuously increase the burden on our health care capacity," she said.

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Carter Williams

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