Later in the post it seems like you're making the jump from 'assume you then touch your face' to 'risk of a grocery trip' but to state the obvious... if you actually don't touch your face before you (properly) wash your hand that solves the whole issue. That seems like it knocks this down into the 'no seriously stop worrying about the risk level' zone, if you can usually sustain it.
It doesn't seem that hard to wash your hands after putting away groceries, say. If I recall, I was not imagining getting many touches during such a trip. I'm mostly imagining that you put many of the groceries you purchase in your fridge or eat them within a couple of days, such that they are still fairly contaminated if they started out contaminated, and it is harder to not touch your face whenever you are eating recently acquired or cold food.
For me, ‘don’t touch your face’ seems way harder than most of the alternatives. I can do it in a short punctuated period of attention but not for any extended period of time.
Seems worth noting the option though I guess.
I haven't looked into this in any detail, but I also don't remember seeing it mentioned anywhere else: is it known whether detecting viral RNA on surfaces is actually correlated with infectiousness? For example, my (limited) understanding of the infection process is that the virus is essentially RNA inside a protein capsule which protects it long enough to infect cells, and any naked foreign extracellular RNA would quickly get broken down by RNases. On surfaces, is it possible that some of the protein capsules have "broken down" such that their RNA is exposed, and therefore much less or even non-infectious?
Mod note: I'm frontpaging this (despite a general policy of not frontpaging covid posts) because thinking numerically about "how risky are fomites" has been a longstanding unsolved problem that seemed particularly important.
My prior for fomite transmission in respiratory viruses is very low: 1988 article on Rhinovirus, hamsters SARS-CoV-2 study, human case series I don't have time to make a serious review though.
I thought fomites was though to have been a significant vector with SARS-1. (i.e. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5519164/)
The authors say "non negligible" though. And it's a simulation study. Besides in the limitations section they acknowledge the absence of literature on many biological parameters.
My household previously made some highly uncertain estimates of the covid risk from bringing random objects that other people have recently been touching into our home, for instance salads and groceries an endless stream of Amazon packages. The official guidance is very vague, e.g. “…not thought to be the main way the virus spreads”. Our bad estimates were fairly low, so we decided to basically ignore it in our covid risk accounting, except for habitually taking some reasonable precautions.
Then the covid rates here increased by a factor of ten, so we decided it would be good to look at it again.
So today I tried to estimate this from this paper (HT Ben Weinstein-Raun and Catherine Olsson) in which a group of researchers swabbed various door handles and trash cans and crosswalk buttons and the like in a small Massachusetts city and measured the covid RNA detectable on them. They also used the amounts they measured to estimate the infectiousness if someone else were to touch the surface and then touch their face.
Here I offer you a draft of an elaborate Guesstimate spreadsheet on the topic, in case you are interested in such things. Hopefully someone is, and will tell me ways that it is wrong, then later I might offer you something reliable. At present, it is probably pretty unreliable, and should only be used insofar as you would otherwise use something even more unreliable. Some non-exhaustive evidence of its unreliableness:
Interesting tentative conclusions so far, rely upon at own risk:
The basic reasoning is this:
I welcome all kinds of comments, e.g. even ‘I want to know why this cell says 27, but I can’t be bothered reading all these things’.