Investigators in the UK reviewed the literature of evidence on how long someone with COVID-19 remains infectious. Their answer: No longer than eight days after symptom onset. Source:
Viral cultures for COVID-19 infectivity assessment – a systematic review (Update 4)
This is important to know because the PCR test for diagnosis of cases can remain positive for up to 83 days. Transmission of infection requires whole live virus – the more the better – not just the fragments of RNA that the PCR tests detect.
The best evidence for ongoing infectiousness is when virus from body fluids can be seen to grow in tissue cultures. Perhaps second best is when tissues or cell cultures are exposed to virus, and microscopic examination shows cell damage.
The commonly used clinical test of COVID-19 infection is called PCR (more accurately, reverse transcriptase polymerase chain reaction to RT-PCR). This test detects fragments of viral RNA, and amplifies them to make them easier to detect. The amount of amplification required for detection is called “cycle threshold” or Ct. If a lot of viral RNA is present, it doesn’t require much amplification. There are many different coronavirus tests on the market. If a test requires too much amplification (i.e., Ct is over 25-35), then it’s probably not a true positive test for SARS-CoV-2; it may be detecting RNA from some other coronavirus, or contamination. Interestingly, my patients have probably been tested with at least three or four different PCR tests over the last six months: the test reports I see never report their cycle threshold.
A false positive PCR test is one where the test is positive but the patient isn’t really infected. If you’re a government who wants a high “case” number, choose a test with a high Ct, over 30-35. (Again, it depends on the particular fragment of RNA being looked for, and other esoteric factors.)
The authors cited a couple cases of COVID-19 in which viral RNA was found in secretions at 78 and 83 days after initial diagnosis. They fully expect, however, that the RNA detected was viral debris, not whole infectious virus.
Remember, this is just one study. It hasn’t been peer-reviewed yet. We’re learning more as each month passes. Stay tuned.
Steve Parker, M.D.