Risk of Exposure

Yesterday’s post estimated that nearly 20 million people have been infected with covid in the US, or about 6 percent of the total national population of 328 million. But not everyone who has ever been infected remains so, and not everyone who is currently infected is contagious.

What’s my risk of exposure when I go out into the world? The best guess is that infected people are contagious for an average of 10 days. Using yesterday’s estimate, around 140,000 people are getting infected in the US every day. Multiply that daily rate by 10 days = 1.4 million people in this country who are currently infected and contagious, or around 0.4% of the American population.

So, that random guy whose path I crossed on my morning walk has less than one chance in a hundred of being covid-contagious: I’ll take those odds.

Still, unlikely isn’t the same as impossible.

What if I were to go to the liquor store tomorrow to replenish our supply of Jameson’s and St. Remy’s, and I find ten other buyers and sellers in the store with me. How likely is it that at least one of those ten is contagious? The probability is 1 – (1 – 0.004)10 = 4.3 percent — still pretty good odds.

How about a trip to the supermarket, mingling with a hundred shoppers and employees? 1 – (1 – 0.004)100 = 33 percent. One chance in 3 of encountering someone contagious at the supermarket — getting dicier.

What if I go to the supermarket once a week for 3 months: now what are the chances of coming across someone who’s corona-contagious on at least one of those shopping expeditions? 1 – (1 – .33)13 = 99.6%:  a near certainty that at some point in my weekly grocery excursions I’ll cross paths with at least one covid-contagious shopper or clerk.

Suppose everyone in America makes a weekly grocery shopping trip: that’s 328 million x 0.33 = 108 million weekly close encounters with contagion in the supermarket aisles.

Of course not every close encounter actually results in transmission of the virus. Besides, things are looking up: a month ago the contagion rate in the US was twice as high, the odds half as good of getting through the day unscathed. Still, it happens often enough for 140 thousand Americans to get infected every day.

But of course my trips to the shops are local. What are the odds here in North Carolina? An average of 20 covid deaths per day divided by 0.6% mortality rate = 3,330 new infections per day; multiply by 10 = 33,300 currently infected and contagious; divide by 10.5 million state population = 0.3% of the NC population is currently contagious. That’s a bit lower than the national rate, the odds of evading infection a little better. But the state rates have been climbing lately, and my city of Durham has experienced a 50 percent higher covid death rate than the rest of the state.

I’m sticking with grocery home delivery.

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