Growing research indicates many COVID-19 cases might not be infectious at all
Elevated 'cycle thresholds' may be detecting virus long after it is past the point of infection.
A growing body of research suggests that a significant number of confirmed COVID-19 infections in the U.S. —
perhaps as many as 9 out of every 10 — may not be infectious at all, with much of the country's testing equipment possibly picking up mere fragments of the disease rather than full-blown infections.
Many politicians, meanwhile — including most state governors in the U.S. — have tied reopening policies to the number of cases detected in the local community, with regions and localities often being permitted to reopen in staggered "phases" only when they have reached successively lower benchmarks of average new daily cases in the area
Numerous institutions, meanwhile, have adopted testing protocols in an attempt to preempt the spread of the virus. American colleges and universities, for instance, have turned to mass testing in order to closely monitor incidences of the disease among students, particularly residential students living on campus.
Yet a burgeoning line of scientific inquiry suggests that many confirmed infections of COVID-19 may actually be just residual traces of the virus itself, a contention that — if true — may suggest both that current high levels of positive viruses are clinically insignificant and that the mitigation measures used to suppress them may be excessive.
At issue is the method by which many COVID-19 tests detect a patient's viral load within a given sample. Polymerase chain reaction tests, which have been widely deployed to determine if individuals are infected with the disease, function by amplifying DNA samples to the point that an antigen can be detected and classified.
The "cycle threshold" is the number of amplification cycles a PCR test goes through before a target pathogen is detected. A lower cycle threshold means that a higher amount of the virus was present in the sample; a higher threshold means the machine had to work harder to detect the virus in the sample, indicating a lower viral load and more likely a non-infectious patient.
According to a rundown of PCR tests compiled by the Foundation for Innovative New Diagnostics, many manufacturers of PCR tests set the cycle threshold cutoff for a positive sample at up to around 40 cycles, a level numerous public health officials believe is guaranteed to return what are effectively false positive results that have detected fragments of the virus.
"I'm shocked that people would think that 40 could represent a positive," Juliet Morrison, a virology professor at the University of California, Riverside, told the New York Times in August.
Health authorities elsewhere have indicated similar skepticism of high-threshold tests. A spokeswoman for Taiwan's Central Epidemic Command Center said in June that the agency only assigns positive cases to samples with Cts of 35 or less, with authorities there believing that any samples with Cts of more than 32 are likely (though not definitely) non-infectious.
A team of researchers at Oxford, meanwhile, wrote in a preprint paper last week that, based on a review of various sample collections, swabs requiring more than 30 cycles were "associated with non-infectious samples."
Binary positive-negative test results — in which cycle thresholds are not considered — will "result in false positives with segregation of large numbers of people who are no longer infectious and hence not a threat to public health," they wrote.
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