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Comment by SomeoneFromCA | Hacker News Reader

There many studies contradicting this. I need to find the links gain (check BNO reinfection tracker, it is one the most recent), but a recent Korean study showed a case of reinfection (or as I am often told to be a "relapse") when a healthy women had shown symptoms again 10 days after recovery, although genetically different. After that in 10 days (or similar time period, do not remember) she showed the symptoms again, third time, with the virus of the first serotype. Also, immunocompromised people keep shedding viruses for months, not only that, the viruses also mutate very rapidly in them.

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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 afterwards and can show up as PCR positive.

This meta analysis only includes studies of multi patients so there's pretty much a guarantee that there are individual case studies that differ from this.

> 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".

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.

Covid is not a DNA virus.

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.

Yes I understand this, and I also believe that meta-analysis needs to be taken with a grain of salt, because it is never as reliable as an original research. In the case of pandemics of a dangerous and highly contagious disease, even individual cases, such as above are important. This is the reason why Asia is so successful in the pandemic management, and the west is a pathetic failure.

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