This is another study exploring PCR testing and infectiousness as determined by culture. (CID Paper) Based on work at a lab in France, the authors examined culture positivity in 3790 samples. At cycle number 25, 70% of the samples were cultured positive, meaning there was virus capable of replication in the samples. At cycle number 30, this dropped to 20%, and at cycle number 35, it was a mere 3%. Let me further note that just because some replicating virus was present in culture does not mean that the patient actually was or was capable of shedding (exhaling) an infectious dose. At those higher cycle numbers, the viral load was likely very, very low.
I keep harping on this subject because a) for doctors it is important to know viral load as this is clearly related t0 likely clinical progression of disease. Cycle number can be correlated with viral load. Therefore, doctors should be given cycle numbers and/or the calculated viral load. They should also with each result be given the likelihood that the cycle number and calculated viral load actually correlate with infectiousness; b) for public policy and public health decisions, this is extremely relevant information–if people aren’t infectious, they shouldn’t be quarantined, and staff shouldn’t be wasting time contact tracing non-infectious people; c) the public should get data on the distribution of cycle numbers, viral load and likely infectiousness along with the daily case numbers. This should be part of a broad-based effort to present more balanced and accurate data and ending the terrorification program.
Finally, labs decide what threshold to use as a positivity cutoff. I wonder, do hospital labs have any incentive to use high cycle numbers when the hospital gets paid more for every CV-19 case? CMS (the Medicare Agency) and other payers should immediately tell hospital labs to stop using any cycle number above 35 as a threshold for positivity.