This comes from thinking about a five-year-old post on Risk.
The State of Utah has not closed down the schools, and does not demand that students in school wear a mask. To some this sounds like recklessness. It isn't.
We know the populations most likely to carry COVID: they are older people. The young do not get COVID nearly so commonly. In Utah, people 24 and younger account for 35% of the cases, leaving 65% of cases in those older then 24. So if you were to spend time in the general population, are your chances of exposure to COVID greater or smaller than spending time in a classroom?
Utah residents diagnosed with COVID-19 by Age
The answer is smaller if you are in the classroom, surrounded by younger people. Compared to shopping, or going to a movie, the classroom is safer.
So what is the risk? In Utah, the total risk is 325 for every 100,000 citizens over a two-week span. If we call shopping at that risk level, that's a 0.325% risk level. What is the risk in the classroom? 35% of that, or 0.114%, 65% safer than being at the mall.
So what is your overall risk if you shop and go to school?
0.325%. Risks are not additive, nor multiplicative. My total risk is whatever activity brings me the greatest risk. COVID is unaware that you went to the mall before you went to school, so it can't know to infect you because it hates the mall.
If you stay at home and nobody from your family goes shopping, but attend school, then your total risk is 0.114%. If someone goes shopping, your risk goes up, to 0.325%.
The Utah epidemiologist and the State school people realized this, and kept the schools opened. Good for them!
I hope we all manage to learn from this experience.
As a follow-up to this post, two days after I posted this UVU chose to not track or follow or test everyone for COVID. Well-done! That's some good waking up to the reality of risk.