Comparing Person Risk methods with an example

Let’s work through a quick example to compare the different approaches for estimating Person Risk.

Reasonable Rosie lives with one roommate in San Francisco and works from home. Rosie grocery shops twice a week in a surgical mask. Nobody else visits Rosie’s apartment, and she doesn’t hang out with anyone else indoors. She went on 5 separate 1.5 hour masked, outdoor, regular distance (3-feet (1 meter) apart) walks with friends over the past ten days. For simplicity, we’ll assume her roommate does the exact same set of activities that she does.

You’re planning to hang out with Rosie and want to know her Person Risk, so you can know what precautions to take. You calculate:

• Basic method: 5106 Person Risk.
  • Because we estimate the San Francisco prevalence of COVID infection (at the time of writing) as 5106-in-a-million. This would change if prevalence changed.
• Advanced method: 217 Person Risk

  • Remember, here you’re calculating Rosie’s own risk of getting COVID from her activities, in microCOVIDs, which you can then use in calculating your risk of getting COVID from her.

  • Each walk starts with a 14% Activity Risk (for one-time contact per hour) times 1.5 hours, and then gets a decrease of 2x for Rosie’s mask, 4x for her friend’s mask, and 20x for being outdoors. Since she stays 3 feet (1 meter) apart from friends on these walks, there is no additional reduction for distance. We’ll treat the friends as average residents (using the 5106 Person Risk from the Basic Method above). Five walks in the past ten days adds up to 0.14/hr ⨉ 1.5hr ⨉ 5 ⨉ 5106 ⨉ (1/2) ⨉ (1/4) ⨉ (1/20) = 34 microCOVIDs.

  • Rosie also goes to the grocery store twice a week. We estimate this as spending 2 hours per week, about six feet (two meters) away (2x) from 5 random people at a time (each with 5106 Person Risk using the Basic Method), wearing a surgical mask (2x). Let's assume the other people in the store are wearing thick and snug cloth masks (3x, not as protective as surgical masks), but that people are not talking (5x decrease). Rosie’s grocery shopping adds up to 5106 ⨉ 0.14/hr ⨉ 2hr ⨉ 5 ⨉ (1/2) ⨉ (1/3) ⨉ (1/5) = 238 microCOVIDs.

  • So Rosie’s errands plus her walks gives her a risk of 238 + 34 = 272 microCOVIDs (or 272-in-a-million chance of catching COVID).

  • If Rosie’s roommate does the same things (two hours of grocery shopping and five walks with friends per week), then Rosie’s roommate’s risk of getting COVID, in microCOVIDs, due to sources other than Rosie, is the same: 272. Multiply this by the 40% Activity Risk of being a roommate and you learn that Rosie’s roommate poses a risk to Rosie of 0.40 ⨉ 272 = 109 microCOVIDs.^[1]

  • So the total COVID risk for Rosie, based on her behaviors, is 238 + 34 + 109 = 381 microCOVIDs. Now you can use this number as the “Person Risk” when you’re calculating your own chance of getting COVID from Rosie.

One thing you will notice is that when we re-compute Rosie's Person Risk via the advanced method, we get substantially smaller numbers. This is because the Basic method assumes Rosie is about average, which is not true. In fact, she is being about ten times more cautious than the average person in her geographic area. The more you know about a person’s behavior, the more accurate your estimate can be. In some cases it might go up, in other cases it might go down.

With a total COVID risk of 381 microCOVIDs, Rosie is being much more cautious than average!^[2] It’s also possible we’re still overestimating her risk, even using the Advanced Method. For example, if her friends are similar to her, then they are probably more cautious than average as well, which would reduce her COVID risk from socializing.

In our calculator we’ve provided some Person Risk Profiles that were made using the Advanced Method. You can use them as a starting point to create your own calculations.

Now that we’ve looked at Person Risk, we can combine it with Activity Risk to get the cost in microCOVIDs of a given activity. Hooray, you made it!