Post #1345: William and Mary COVID-19 trend, over one new case per day

Posted on December 11, 2021

Source:  Calculated from William and Mary COVID-19 dashboard.

Not sure quite what to make of that.

New case rates have ticked up in Virginia as a whole, consistent with a “winter wave” of COVID (Dark line on the graph below).

Source:  Calculated from NY Times Github COVID-19 data repository.

I guess that’s not a particularly worrisome level of new cases.  And there’s not much that you can do anyway, as the semester is nearly over.

All the things we did last year at this time

With this endless pandemic, I find it helpful to look back and see how things have changed for the better.  In this case, it’s interesting to compare the end of the semester this year to last year.

What I find most striking is that this year, nobody is making noises about the need to quarantine returning college students.  Last year, that was very much in the news (Post #988, November 2 2020.)  Nor am I seeing recommendations for COVID-19 testing prior to return to home, which I believe was more-or-less the norm last year.

This year, with a much more muted winter wave of COVID, and vaccination for more-or-less everyone who wants it, I guess the risks are just that much lower.  Or we’re that much more tired of dealing with it.  Or some combination of the two.

That said, end-of-semester is a good time to make some ballpark estimates of risk of spreading disease.  Not to be morbid, but to get some sort of a handle on the odds.  Last year, everybody was offering advice about quarantining your college student.  This near, nobody’s saying anything.  Either way, you might want to step back and ask the simple question:  Given the rate of new cases on campus, what are the odds that my kid is going to come home asymptomatic but infectious with COVID-19?

The above is an abbreviated version of the calculation given in Post #988 cited above.  It isn’t rocket science.  You start with new cases showing up on the William and Mary campus at a rate of 19 new cases per 6600 on-campus students for the last two weeks.  From that, you take a guess as to how many students will be leaving the campus in an infectious-but-asymptomatic state.  That guess will depend critically on how many days you think the average infected student remains infectious-but-asymptomatic.  I have assumed a generous 8.6 day average period (calculation shown in Post #988).

Anyway, my best guess is somewhere around 1 in 600.  Or so. FWIW.  One in 600 will return home in an asymptomatic but infectious state.  Again, not rocket science.  That’s the same as saying that a bit over a week’s worth of new cases will walk off the campus undiscovered.

There’s one big problem with applying last year’s calculation to this year’s data.  Last year, there was campus-wide testing, and it presumably identified asymptomatic cases.  This year, there was no systematic effort to test and find asymptomatic cases.  This year, we ought to add in a fudge factor for those undiagnosed, asymptomatic cases.  Best guess, historically, 40 percent of COVID-19 cases have been asymptomatic.  So you’d not-quite-double the estimate, to account for the missing asymptomatic cases.

On the other hand, an infected student doesn’t necessarily translate to spread of disease.  Even in the pre-vaccine era, there was no guarantee that an infected household member would spread it with in the household.  That “secondary attack” only occurred in about 15 percent of cases.  Now, with vaccines, I’d have to guess that only a tiny fraction of those students will go on to spread COVID at home.

The upshot of that last bit is that the likelihood of my kid coming home and spreading COVID-19 in the household is much less than the one-in-600 cited above.  One in several thousand, at a guess.

Even as vague as all that is, I find it helpful to ballpark the risk.  Best guess, there’s something like a one-in-several-hundred chance that your kid is going to come home with COVID-19, and be infectious, and not know it.  And the odds of that actually leading to an infection within your household are one in several thousand, best guess.

It’s not one-in-a-million, but it’s not one-in-a-hundred either.  Scale your response (if any) accordingly.  For our part — vaccinated and boostered — we’re just not even going to give it a thought.  YMMV.


Students return to campus on 1/26/2022, or seven weeks from now.  Based on the South African experience, and what is now predicted for Great Britain,  Omicron will be the dominant strain in the U.S. long before then.  It should take a month, at the outside, to go from first discovered cases to dominant strain.

My point is, this year, students are leaving the campus under Delta.  They’ll be returning to the campus under Omicron.

Oddly enough, for a guy who’s clearly been worrying about this a lot, I don’t think Omicron is going to be a problem.  I think it’s going to be an asset.  My prediction is that it’s going to spread like crazy, but very few individuals are going to get very sick from it.

(This, despite the fact that two vaccine doses alone does not protect you from an Omicron infection.  Two doses might give you some protection against a serious case (e.g., one requiring hospitalization).  But with two vaccine doses only, people seem to mount essentially no antibody response to Omicron.  You need three doses (two plus booster), or two doses plus prior infection, to generate significant antibody response to Omicron.)

In any case, my projection of widespread-but-mostly-harmless Omicron is based on my reading of the data out of South Africa, which you can see in my just-prior posts.  The widespread part is pretty much a given.  The “mostly harmless” is my reading of some tenuous data.  I should be on firmer ground there in a week, so when I update these numbers a week from now, I’ll provide my best guess on how Omicron is going to play out in the U.S.

However it plays out, that’s what we’ll be facing when students return to campus next year.