https://medium.com/@vernunftundrichtigkeit/coronavirus-why-everyone-was-wrong-fce6db5ba809

When the ultra sensitive PCR tests are done on an immune person (a healthy person with antibodies), it is not a virus that is detected, but a small shattered part of the viral genome.
The test comes back positive for as long as there are tiny shattered parts of the virus left. Even if the infectious viruses are long dead, a corona test can come back positive, because the PCR method multiplies even a tiny fraction of the viral genetic material enough to be detected.
That’s exactly what happened, when there was the global news, even shared by the WHO, that 200 Koreans who already went through Covid-19 were infected a second time and that there was therefore probably no immunity against this virus. The explanation of what really happened and an apology came only later, when it was clear that the immune Koreans were perfectly healthy and only had a short battle with the virus. The crux was that the virus debris registered with the overly sensitive test and therefore came back as “positive”. It is likely that a large number of the daily reported infection numbers are purely due to viral debris. The PCR test with its extreme sensitivity was initially perfect to find out where the virus could be. But this test can not identify whether the virus is still alive, i.e. still infectous. Unfortunately, this also led some virologists to equate a PCR test result with viral load, i.e. the amount of virus someone can breathe out.” No symptoms, no viral load.