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> This study says live virus has only been confirmed to be seen for at most 9 days. However, (presumably inactive) virus particles can be shed for months afterw

> This study says live virus has only been confirmed to be seen for at most 9 days. However, (presumably inactive) virus particles can be shed for months afterwards and can show up as PCR positive. The paper cites 34 replication cycles as the lower bound to obtain viable viral cultures. Whether a PCR test is reported as "positive" depends on the chosen replication cycle threshold (Ct value). As far as I'm aware, this "magic number" is usually chosen to be 35. Beyond that, it is possible to do more cycles to demonstrate the existence of trace amounts of PCR fragments, but such a test would not be considered "PCR positive".

gregwebs 6y ago
The issue is that a PCR test does not actually tell us if a virus particle is live or inactive. A shed virus particle can still have intact RNA for the region tested by PCR. There are published case reports where someone has tested PCR positive during an infection, then PCR negative, and then PCR positive again later and the explanation given has been that they did not get re-infected and their infection did not flare back up but just that virus had been continually shed and the later test was able to capture that. edit: s/DNA/RNA. The PCR process actually operates on DNA so the virus RNA is converted to DNA as the first step of RT-PCR.
- SomeoneFromCA 6y ago
  Covid is not a DNA virus.
- westwing 6y ago
  The PCR thresholds are chosen to minimize false negatives, not false positives. A sample that required 30+ replication cycles is unlikely to come from a person that is infected/infectious, yet it would be counted as "positive". The thresholds are derived from experiments under lab conditions, providing a safe lower bound. That does put into question the validity of counting PCR test positives as a benchmark of viral load among the population, but as a safety measure for potentially infected individuals, the trade-off is worthwhile.

gregwebs 6y ago

The issue is that a PCR test does not actually tell us if a virus particle is live or inactive. A shed virus particle can still have intact RNA for the region tested by PCR. There are published case reports where someone has tested PCR positive during an infection, then PCR negative, and then PCR positive again later and the explanation given has been that they did not get re-infected and their infection did not flare back up but just that virus had been continually shed and the later test was able to capture that. edit: s/DNA/RNA. The PCR process actually operates on DNA so the virus RNA is converted to DNA as the first step of RT-PCR.

SomeoneFromCA 6y ago
Covid is not a DNA virus.
westwing 6y ago
The PCR thresholds are chosen to minimize false negatives, not false positives. A sample that required 30+ replication cycles is unlikely to come from a person that is infected/infectious, yet it would be counted as "positive". The thresholds are derived from experiments under lab conditions, providing a safe lower bound. That does put into question the validity of counting PCR test positives as a benchmark of viral load among the population, but as a safety measure for potentially infected individuals, the trade-off is worthwhile.