A Key to Reopening Schools -- and Keeping Them Open

Sean C. Lucan, MD, MPH


March 17, 2021

As schools reopen, periodic screening for SARS-CoV-2 may add a layer of protection. However, choosing the wrong COVID-19 screening test could force schools to close again.

I have written previously about why once-weekly testing by polymerase chain reaction (PCR) is a losing strategy; weekly PCR tests give false reassurances and lead to needless quarantines and shutdowns. What about an alternative approach?

An invaluable figure by Michael Mina and colleagues tells you almost everything you need to know about SARS-CoV-2 screening tests. A modified version may offer even more clarity (Figure 1).

Infection Progression

Let's start with the "amount of virus." When someone is initially infected with SARS-CoV-2 (day 0), a miniscule amount of virus rapidly starts to replicate inside of them until the virus is so abundant that it can no longer be contained. The "infected" person becomes "infectious" (contagious) and can now spread the virus to others.

The transition to becoming contagious generally occurs around day 3. By around day 5, infected individuals usually hit their contagiousness peak. At that point, individuals' immune systems start to contain the situation. So from day 5 onward, the amount of virus begins to drop.

Ultimately, after about a week of contagiousness (ie, by about day 9), the virus is essentially vanquished. Viral particles remain for weeks or even months but, like slain enemies on a battlefield, they no longer pose a threat to others.

So, what does all of this mean for screening tests?

Two Testing Strategies

In regard to screening-test detection level, there is no question that PCR is "better." It can detect virus very slightly earlier, and for much longer (Figure 1). However, what we really care about is detecting contagious virus.

For detecting contagious virus, antigen tests perform just as well and do not create confusion by detecting virus that is no longer a threat.

Consider two screening strategies: once-weekly PCR vs thrice-weekly antigen testing (Figure 2).

On the basis of cost alone, thrice-weekly antigen testing (at about $5 per test) is actually less expensive than weekly PCR testing (at about $150 per test). Not only is antigen testing less expensive, but it also is actually the better option. Here's why.

Problems With PCR

In Figure 2, day 0 is a Sunday. Let's say a student gets infected at a weekend playdate. Although the virus starts to replicate inside the student, the level will still be below the threshold for detection when a PCR test is done on Wednesday. If the test results come back on Friday (a typical turnaround), they will be negative — true for how the student was on Wednesday; false for how the student is on Friday.

The now-infectious student is thus falsely reassured, along with classmates, teachers, parents, and administrators. The student is allowed to stay in school and to return the following week.

When the infected student is again tested on Wednesday, the results on Friday come back positive — true for Wednesday but false for Friday (in terms of contagiousness). So, after an entire week of being contagious in school, the student is now sent home to self-isolate and the classroom is quarantined.

Nobody benefits. Everybody loses. The expensive, sensitive test fails.

Imagine now that, because of false positives, another classroom winds up shutting down. As it happens, that class's teacher was in a situation like that shown for Week 3 in Figure 2 (PCR truly positive for virus but falsely positive for contagiousness). In other words, the teacher had a distant infection from which she has long recovered. She is no longer a threat. But given multiple classrooms now with "positive" tests, it appears that the school has an "outbreak." The school shuts down.

The Alternative

What would have happened with antigen testing? Had the play-date–infected student been screened with thrice-weekly antigen testing, that student would have been detected on Friday just as he/she was becoming contagious. The results would have been known almost immediately (within 15 minutes). The student would have been sent home to self-isolate, and close contacts would have been sent home to quarantine.

Additionally, the PCR-positive teacher would have tested negative with an antigen test. The class would not have had to quarantine, and there would have been no perception of an "outbreak" at the school. The school would have remained open, safely.

Daily antigen screening would be even more effective. If everyone were screened daily, before entering classrooms, positive cases could be identified (and isolated) prior to exposing others.

But even without daily screenings, a strategy of antigen testing multiple times per week would allow threats to be discovered right as they are emerging.

We can get kids learning and teachers teaching again — in person. Safely and affordably. We just need to avoid using the wrong screening test.

Follow Medscape on Facebook, Twitter, Instagram, and YouTube


Comments on Medscape are moderated and should be professional in tone and on topic. You must declare any conflicts of interest related to your comments and responses. Please see our Commenting Guide for further information. We reserve the right to remove posts at our sole discretion.