Only a little more than a month after World Health Organization (W.H.O.) declared the novel coronavirus a pandemic on March, 11, researchers in France revealed an important correlation between Cycle threshold values and the contagiousness of SARS-CoV-2.
Cycle threshold (Ct) is “…the number of cycles needed to amplify viral RNA to reach a detectable level.”(1)
In other words, Ct values are the metric used in diagnostic testing to reveal whether someone has traces of a virus in their system or not. The most important aspect of a Ct value is the fact that it is inversely proportional to the viral load (the concentration of virus in blood). Which means that the more cycles it takes to detect viral RNA, the smaller the viral load will be.
On April 27, 2020, researchers from the Méditerranée Infection University Hospital Institute in Marseille, France, observed the relationship between a patient’s Ct value and their chances of being contagious.
To complete their research effectively, they placed samples (which had tested positive for a SARS-CoV-2 and been through the PCR testing process) on cultures to effectively measure the positivity rate of the culture.
What they found was that “Samples with Ct values of 13-17 all led to positive culture. Culture positivity rate then decreased progressively according to Ct values to reach 12% at 33 Ct.”(2)
This means that samples with higher Ct values were unable to produce a positive rate on the culture. Which in turn proves that positive tests found through Ct values > 33 are not contagious.
Their consensus says, “we can deduce that with our system, patients with Ct values equal or above 34 do not excrete infectious viral particles.”
Dr. Anthony Fauci himself stated in a July 2020 interview with TWiV (This Week in Virology), that “What is now sort of evolving into a bit of a standard, is that if you get a cycle threshold of 35 or more that the chances of it being replication competent are miniscule.”(3)
The researchers go on to say, “We propose that each center perform its own correlation between culture results and viral RNA load from patients’ samples.”