Yesterday William and Mary reported about 150 new test results, and two additional positive cases. By my count, they still need test results from maybe 500 more students.
Maybe the easiest way to put the past few days of results into perspective is to compare the test positivity rate to some “baseline”. That’s shown in green below. Just before this outbreak hit, for the semester, 0.8 percent of on-campus COVID-19 tests on students were positive. In these last batches of tests, about 1.2 percent were positive. So they are still above that historical baseline, but not hugely so.
And so, right now, it appears that the risk of a new COVID-19 infection for a W&M student is only modestly higher than it has been all along. Call it 50% higher than the average risk so far this semester.
In addition, recall that the W&M administration said that 80% of the St. Patrick’s outbreak cases were for students living off-campus. Assuming that remains true, that works out to be a sharply elevated risk for students in off-campus housing, and a relatively modest risk for students living on campus.
Can you tell how much continued spread of COVID-19 is occurring on campus?
The short answer is, no, you can’t tell.
In terms of looking forward, do the most recent batches of tests tell me much, other than the fact that this hasn’t spun out of control? What I’m really after is, can I guess how many actively infectious students are currently circulating within the W&M student population?
The answer to that is no. But I think it’s worth documenting what the CDC says regarding isolation after known infection.
The CDC isolation guidelines can be found at this link. Starting from that, the first question is: How long do individuals remain capable of infecting others with COVID-19? CDC says this:
For patients with mild to moderate COVID-19, replication-competent virus has not been recovered after 10 days following symptom onset.
In other words, if you had symptoms, and didn’t require hospitalization (which is what gets a case classified as “severe”, instead of mild or moderate), you would be absolutely guaranteed unable to spread COVID-19 10 days or more after symptom onset. For the simple reason that you would no longer be be housing any live (replication-competent) virus.
But that’s too stringent a test — absolute lack of live virus. In fact, the actual period of infectiousness actually seems to be somewhat shorter than that. The CDC goes on to say this:
In a large contact tracing study, no contacts at high risk of exposure developed infection if their exposure to a case patient started 6 days or more after the case patient’s infection onset.
You have to read the cited reference to see that “infection onset” was actually symptom onset. And so, based on that one contact-tracing study, practically speaking, you are probably unable to spread COVID-19 6 days or more days after symptom onset.
In the end, the CDC guidelines stuck with 10 days. And so, if you become infected, and don’t require hospitalization, you’re supposed to isolate yourself for 10 days following onset of symptoms. As long as the symptoms have resolved, and it’s at least a day since you’ve had a fever, you’re good to go at that point.
And if you don’t have symptoms, the rule is, 10 days from the date of the positive test. Individuals without symptoms, but who have a positive test, are still supposed to go into isolation.
But, unfortunately, that’s not the end of the story. What we actually have information on is students who test positive. And you can test positive for a long time after you’ve fully recovered. And here’s what the CDC has to say about that:
Recovered patients can continue to have SARS-CoV-2 RNA detected in their upper respiratory specimens for up to 12 weeks after symptom onset.
This is why the CDC recommends that you don’t get re-tested for COVID-19 in the 90 days following your illness, as long as you show no symptoms of COVID-19. There’s too high a likelihood of a false-positive DNA test from the dead virus fragments that remain in your respiratory tract.
The implication for W&M is that the positive test results that are currently being reported by W&M are some mix of people who have an active infection (symptomatic or asymptomatic), and people who have long-since recovered from an asymptomatic infection but have dead virus remaining in their system.
This assumes that anyone who had symptoms had the good sense to get tested. That’s why it’s only the recovered-asymptomatic cases that would be caught this way.
Unfortunately, asymptomatic cases are common. Dr. Fauci is reported to have stated that 40 percent of U.S. cases are asymptomatic. It’s almost certain that this proportion would be higher among college-age individuals.
Normally, out in the community, that false-positive-for-recovered-patients effect hardly matters. Most people only get tested for a good reason. Either they have symptoms, or they have a known exposure to an infected individual. And so, if they get tested and have a positive test, the odds are pretty good that they were infected recently and so can be presumed to have an active infection.
But here, with “census” testing, W&M is testing everyone, regardless of symptoms or known exposure. So those tests will pick up some mix of people who might plausibly be actively infectious, and some who are long-recovered asymptomatic patients with enough dead virus in their respiratory tract to trigger a positive test.
I don’t think there’s even a way to guess at the quantities there. Among other things, as time goes on, the chance of a previously-infected individual trigger a positive test falls. (You can see that in Post #859 if you have an interest.)
Let me sum this up.
Almost all the students that were actually infected on St. Patrick’s day should be well beyond the point of being infectious. Those students are no longer spreading COVID-19. The median lag between infection and symptom onset is four to five days, and then the best evidence (via CDC above) says that non-hospitalized individuals rarely remain infectious no more than six five days after symptom onset. That’s a total of maybe 11 10 days, and we’re now 17 days past St. Patrick’s day. Those people are now effectively out of the picture, whether or not W&M found them via testing and isolated them.
But we can’t look at the current rate of positives — about 1.2 percent of those tested — and know whether or not those people were infectious at the time of testing. The tests will pick up those who have an active infection at the time of the test, and some fraction of asymptomatic-but-recovered individuals who retain enough dead virus to trigger a positive on the test.
A final, unavoidable complication is that the PCR test itself has a fairly high false-negative rate (Post #859). Some more-or-less unknowable fraction of the students with negative tests actually had an active COVID-19 infection at the time of the test. They just didn’t find enough RNA on the test swab to trigger a positive.
In the end, all you can say is that the it sure looks like the outbreak is winding down. But I don’t think there’s any good way to quantify just how rapidly it is winding down. Near as I can tell, there is no reasonable way to guess how many students are still circulating and spreading COVID-19 at the moment.
