By backwards tracing I mean figuring out where infection took place. Tracing backwards up an infection tree is much easier because the further up the tree you go, the fewer branches there are. In your case, for instance, there are probably at most a dozen points of likely exposure in the given time period, even if you assume that casual short-term exposures while shopping might have been enough. Once you start moving down the tree there are literally hundreds: everybody in the Petco, the UPS office, the coop, the bike store, the drug store, etc. When the virus is new to a community it might actually be possible to notify everybody who ate in a certain restaurant on a given day that they might have been exposed. But when it is everywhere, the logistics are impossible. Suppose it was actually possible to contact trace every one of the 5,794 people that were confirmed positive in Minnesota yesterday (which it obviously is not, but let's just pretend). How many places did those 5,794 people go in the last week where they might have exposed someone? There's probably not a restaurant or bar or grocery store in Minnesota that didn't have at least one infected person in there in the past week. So what we really need to know at this point is how likely is it that 1 (or 2, or 10...) people in a grocery store or bar will spread the infection to someone else?
Probably the most useful information that we have about how the virus actually spreads is from super-spreader events, which is info you get from backward tracing. It has been obvious since the church-based outbreak in Seoul that aerosol spread was involved. The infamous choir practice in Seattle made it clear that singing (and presumably other forms of loud verbal expression) hugely accelerated the infection rate and that liberal use of hand sanitizer did nothing to stop it. And the dedicated testing stations for George Floyd protesters in Minneapolis was a brilliant way to gather data on the likelihood of outdoor spread with moderate mask use, even when people are shouting.
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Probably the most useful information that we have about how the virus actually spreads is from super-spreader events, which is info you get from backward tracing. It has been obvious since the church-based outbreak in Seoul that aerosol spread was involved. The infamous choir practice in Seattle made it clear that singing (and presumably other forms of loud verbal expression) hugely accelerated the infection rate and that liberal use of hand sanitizer did nothing to stop it. And the dedicated testing stations for George Floyd protesters in Minneapolis was a brilliant way to gather data on the likelihood of outdoor spread with moderate mask use, even when people are shouting.