RE: Saliva-Based Antigen Testing Is Better Than The PCR Swab15 Sep 2020 12:27
Part 3
Antigen Testing Is Maximally Sensitive When People Are Maximally Infectious
It’s a gross (but useful) simplification: infectiousness depends on “viral burden.” The more virus you have in your airways, we believe the more likely you are to transmit to others. Viral burden rises steeply in the days immediately following exposure. The typical infected person begins to shed large, transmissible quantities of virus starting (on average) around Day 3 after exposure. Symptoms, if they emerge, will follow around Day 5. By Day 8, whether or not symptoms have emerged, viral burden and infectiousness are both in steady decline. All these numbers will vary from person to person, of course, but the general lesson is this: there is a roughly 5-day window of maximum transmissibility, stretching from Day 3 to Day 8 following exposure.
Another useful simplification: viral burden is also associated with the likelihood that an antigen-based test will pick up the presence of infection. The more virus in the airway, the greater the likelihood of a positive antigen result. What that means is that—conveniently—there is a high degree of overlap between the period of maximal infectiousness and the period during which the antigen-based test is most likely to yield positive results. And, just as importantly, antigen-based testing is highly specific: unlike its PCR counterpart, the antigen test isn’t fooled by faint signals of lingering viral matter outside the period of infectiousness.
For purposes of surveillance screening, those antigen-based negatives worrying the FDA aren’t false negatives at all; those are true negatives for disease transmission. Far from being problematic, in the context of outbreak containment, the antigen test’s limited window of sensitivity is a major asset. The antigen test is ideally suited to yield positive results precisely when the infected individual is maximally infectious.
Contrast that with PCR, whose performance barely depends at all on viral load. In principle, PCR can detect just a few copies of viral RNA. PCR starts to pick up the scent on Day 2 and continues to return positive results for as many as 6-12 weeks after exposure. That’s great if you’re interested in knowing if someone was infected in the past several weeks to months; but it’s a problem if you are interested in knowing if someone is infectious right now. The PCR continues to produce positive results long after a person has ceased to pose any real risk of transmission to others, some data say up to 12 weeks longer!
As a test of infectiousness, the PCR test is far too prone to false positives. These false positives clog up the testing and contact tracing infrastructure and needlessly ground a lot of people who pose no transmission risk to others. PCR is the wrong tool for the surveillance job.
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