We know this because the folks who developed the test the WHO recommends to use have written about their development process:
We downloaded all complete and partial (if">"400 nt) SARS-related virus sequences available in GenBank by 1 January 2020. The list (n"="729 entries) was manually checked and artificial sequences (laboratory-derived, synthetic, etc), as well as sequence duplicates were removed, resulting in a final list of 375 sequences. These sequences were aligned and the alignment was used for assay design (Supplementary Figure S1). Upon release of the first 2019-nCoV sequence at virological.org, three assays were selected based on how well they matched to the 2019-nCoV genome (go to Figure 1). The alignment was complemented by additional sequences released independently on GISAID (https://www.gisaid.org), confirming the good matching of selected primers to all sequences.
The selected oligonucleotide assays, each specific for a certain snippet of the SARS-CoV-2 virus RNA, were then tested for their sensitivity and chemical stability.
They were also tested for cross-reactivity with other viruses:
Cell culture supernatants containing all endemic human coronaviruses (HCoV)"229E, "NL63, "OC43 and "HKU1 as well as MERS-CoV were tested in duplicate in all three assays (go to Table 2). [..] Additional undiluted (but not quantified) cell culture supernatants were tested as summarised in Table 2. These were additionally mixed into negative human sputum samples. None of the tested viruses or virus preparations showed reactivity with any assay.
In total 297 clinical samples with 23 different human virus types in them were tested. The newly developed assays developed to find only SARS-CoV-2 reacted with none of those.
The PCR test for SARS-CoV-2 has a high specificity. It can not detect other types of viruses.
There are additional safety procedures to avoid false tests.
Each test run of typically 90 to 120 samples will include one quality control sample with a known quantity of the SARS-CoV-2 virus RNA. It will also include one quality control sample that is guaranteed to contain no virus RNA. At the end of each run the results of both quality assurance samples will be compared with the expected value. If there is any mismatch the whole run it will be repeated with fresh sample extracts.
When the laboratory machine runs the SARS-CoV-2 PCR test it also will also note the number of cycles it needed for each sample to first detect a reaction. That will typically be in the 20-30 cycles range. If a detection is only made towards the end of the 40 cycle program the machine will note this and alert its operator. Tests which only show positivity above 35 cycles will usually get repeated as such a low reactivity may point to a potential sample contamination.
Where a coronavirus test can go wrong is at the point of sample taking. The swab that is used may not have picked up enough gunk to catch a significant number of viruses. The PCR test will then show the person as negative even when it has caught SARS-CoV-2. There can also be bureaucratic errors where the sample is attributed to the wrong person. The test protocols are designed to prevent this and such cases are rare.
When a person gets infected with SARS-CoV-2 and starts to reproduce the virus its numbers explode to billions of copies per milliliter. When the immune system starts to defeat the virus the number will go down. Debris of dead virus may still be in the body four to five weeks after the infection onset even when the person is no longer infectious. The graphic below shows which test reacts at which stage of an infection.
A person that is tested PCR-positive for SARS-CoV-2 will have been infected with the virus. There is no other way to pick up the RNA snippets the PCR test is looking for. But that person may not have developed COVID-19 symptoms and may no longer be infectious. We do not know this for sure. Tests to find out if a person still spreads viable viruses take a several days and require a lot of manual labor in high security laboratories. These can not be done for everyone.
To recap:
- The PCR test for SARS-CoV-2 is highly specific for that virus and does not detect any other ones.
- A positive PCR tests demonstrates that the person has or has had the virus.
- We have no practical way to tell if that person, even when it shows no symptoms, is still infectious and spreading the disease.
The only way to prevent new infections coming from a PCR-positive person is to isolate that person. After 10 days the immune system of most people will have defeated the virus. (That a significant number of people are still ill at that point is the consequence of an exaggerated immune reaction to the virus, not of the virus itself.)
It is sad that an otherwise useful site like Global Research is spreading such false information about the Coronavirus pandemic. Chossudovsky should stick to writing about social issues. He obviously lacks the basic hard science knowledge that is necessary to understand how PCR tests work.
Spreading such unqualified statements during a pandemic like Chossudovsky's piece does is highly irresponsible.
That is the reason why I delete comments at this site [Moon of Alabama] which spread similar nonsense.
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