Post #1243: To test or not to test?

Posted on September 10, 2021


Source of counts:  William and Mary COVID-19 dashboard.

Today’s case counts from William and Mary show a steady but high rate of new cases reported on campus.  Today’s 42 new cases is not materially different from yesterday’s 40 new cases.

Pre-move-in screening

William and Mary did not screen the entire student body for COVID-19 infections prior to return to school this fall.  But last year, they did.  My wife tells me the decision not to test is now being strongly criticized.

The point of this post is to explain that the decision to test everyone prior to return to campus is not as cut-and-dried as you might think.

Let me get to the punchline.  They tested everybody last year.  They didn’t do that this year.  What changed?  In a word, vaccination. 

This first part of this is easy, and I’m including it in this post.  The first part just documents, with citation as to source, that William and Mary followed the most recent CDC guidance in not screening vaccinated students prior to move-in.

What is current CDC guidance in this area?  In a nutshell,  screen the unvaccinated only.

But in the process of that, I’ll document how the CDC guidance has changed.  Half a year ago, CDC guidance was “screen everybody”.  I think it shows the extent to which reasonable people might disagree about the right course of action.  Screening everybody was and probably is a rational way to go about return-to-campus.  The CDC doesn’t rule it out.  But it’s not what CDC recommends right now.

The harder task is to explain why that CDC guidance makes some sense.  That’s for a separate posting, and has to deal with the false negative and false positive rates of COVID-19 tests.

My bias is that I wish they had tested everybody.   But I’ll admit that’s just a gut reaction, based on no hard data or logic.  My only reasoning is that, last year, that seemed to help.  I’m now putting that aside in favor of trying to understand what the experts said, and, separately, why they said it.

William and Mary followed CDC guidance.

First, distinguish between “screening testing” or “routine testing”, and “diagnostic testing”.  The former means testing everybody.  For example, testing every student before their return to campus.  The latter means testing for cause — testing people who have symptoms or who have a known exposure to an infected individual.

You are always going to do diagnostic testing.  The question is, under what circumstances should you do screening testing?

Virginia and CDC guidance on testing

The guidance to Virginia colleges and universities keeps changing. To know what William and Mary was supposed to have done in the Fall of 2021, you have to find the most recent applicable expert guidance.

Virginia provided its own explicit guidance for the initial re-opening of colleges in 2020.  This was, I think, partly a legal requirement consistent with the emergency executive orders issued by the Governor.  In order to be allowed to re-open, schools had to demonstrate that they were following the guidance issued by the Commonwealth.

That’s no longer true, and now, guidance to Virginia colleges comes from the Virginia Department of Health.  And VDOH simply says, follow what the CDC says to do.  So it all boils down to what the CDC’s experts say universities should do.

Prior to mid-summer 2020, CDC guidance to colleges about testing said flatly and unambiguously, in several places, that colleges should have screening testing in place, and colleges should screen everyone prior to return to campus.  That guidance is “Interim Guidance for SARS-CoV2 Testing and Screening at Institutions of Higher Education (IHEs), updated Mar. 17, 2021“.

This older guidance is still on-line, and still referenced by a lot of official documents, including CDC web pages.  You have to understand that newer guidance supersedes older guidance at the CDC.  So if newer guidance is issued, you are supposed to follower that instead.

The CDC’s July 23 updated guidance to colleges clearly differentiates vaccinated from unvaccinated students.  Here’s the CDC’s guidance to colleges, updated as of July 23, 2021.


The first bit of CDC guidance is clear:  Screening testing of vaccinated individuals is not needed.  The second one is a little convoluted, but if you read it carefully, it says that screening testing of unvaccinated individuals is needed.  But that screening of vaccinated individuals is not.

The guidance does not bar screening testing of vaccinated persons.  It just says that it is not needed.  In sum, the most current guidance says that colleges should screen unvaccinated individuals, but they don’t need to screen vaccinated individuals.

William and Mary’s testing plan

In short, William and Mary did what the most recent CDC guidance says to do.

They required pre-move-in screening testing for all unvaccinated individuals.  And they are continuing with routine testing until those individuals are vaccinated.   There is no screening testing requirement for vaccinated individuals.  And that’s consistent with the most recent CDC guidance.

Source:  William and Mary

An unnecessary comment on the entire scope of William and Mary’s COVID-19 plan for Fall 2021.

I just read through all of William and Mary’s COVID-19 plans for the Fall 2021 semester, and here’s my summary judgment.  They did nothing that dipped below the standards and recommendations set by the CDC in recent months.  But they also did little to exceed them. 

By contrast, last year, W&M went above and beyond the recommendations in place at the time.  At that time, the Commonwealth did not require pre-move-in-testing, and in fact discouraged the use of screening tests in the university setting (due to the presence of false positives from COVID-19 tests.)

The lack of pre-move-in testing is the one aspect of the current semester’s plan that seems to be drawing the most flak.  But in the context of testing, they did two other things that show less-than-full effort.  First, it’s harder to get a test.  If you’re not symptomatic or have significant recent exposure, you can’t get a free test through William and Mary.  Second, they dropped wastewater testing.  Realizing it’s not exactly quantitative, it struck me as a low-hassle way to monitor the presence of COVID-19 on campus.

In short, I think that W&M — following in the footsteps of the CDC — may have relied too much on the efficacy of vaccines.  As I read it, the plan at the start of this semester was universal vaccination and some common-sense masking requirements.   (And screening the unvaccinated until they got vaccinated.)   That’s about the extent of it.

Even that is hardly irrational.  Just look at the most recent numbers from the Virginia Department of Health (below).  Despite all the scare-mongering reporting you will hear, if you look at at a clean set of data, you can see that the vaccinated really are different from the unvaccinated.  And that, as long as you don’t increase your exposure to COVID-19, vaccine results in a dramatic reduction in risk of infection.  Exactly as the vaccine clinical trials demonstrated

Source:  Commonwealth of Virginia COVID-19 dashboard, accessed 9/10/2021.

In theory, the current 93% vaccination rate should be (just barely) enough to contain outbreaks of the Delta variant.  On its own, with no other COVID-19 hygiene in place.  Most common estimates for the “herd immunity” level with the Delta variant come out somewhere around 90%.  (My own came in at 91%).  It depends, in part, on what you think the efficacies of the vaccines are, against COVID-19.

But people forget what the “in theory” clause means here.  Among other things, “in theory” requires that William and Mary students have no more than average exposure to situations in which COVID-19 can be transmitted.  Throw in some parties and some dorm living, and I’m not at all sure that the “herd immunity” calculations done for the average of the U.S. population and the Delta variant really apply to college students.

At best, you can say that there was little margin for error this year.  And unmasked social interaction on and off campus was enough to push the spread of COVID-19 in the student population.  And maybe, in hindsight, tougher rules up front would have been warranted.

Hindsight is always 20-20.  If I’d been in their shoes, looking at CDC guidance, looking at vaccine impact numbers, and looking at the 93% (96% for campus residents) vaccination rate, I’m not sure I’d have done any differently.