Story at a glance
- April 27, 2020— a research team in France published a study in which they found a link between Cycle threshold (Ct) values and contagiousness of SARS-CoV-2. They found that those who tested positive with Ct values > 33 were unable to spread the virus because the viral load was too low.
- December 14, 2020— a research team from University of Florida found that patients who tested positive for SARS-CoV-2 but remained asymptomatic did not spread the virus to their house members, despite homes being the location with the highest transmission rates.
- On the same day (December 14, 2020)— World Health Organization released their first ‘medical device alert’ regarding PCR assay usage at testing sites.
- January 20, 2021— World Health Organization re-issues their medical device alert, reminding testing sites to manually reset their PCR thresholds to a lower value to avoid misdiagnoses and to closely observe the clinical presentation of positively-diagnosed patients.
- The extensive studies and research show that those who are asymptomatic will test positive with a Ct value > 35, and be non-contagious.
Flashback to April, 2020
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.”
8 months later
However, it was not until December 14, 2020, that W.H.O. released a medical device alert warning testing cites to manually reset their Cycle thresholds to lower values to avoid detecting smaller, insignificant, viral loads.
When the EUA for diagnostic panels first rolled out on February 3rd, 2020, most PCR Cycle thresholds were pre-set to the 40’s, which means that they would have to be manually reset to a lower value by the testing sites themselves.
Interestingly, on the same day (December 14, 2020), a team of researchers from University of Florida published their findings in the JAMA Network Open Journal.
What they found was that those who tested positive for SARS-CoV-2, yet were asymptomatic, did not spread the virus to their house members.
Their research says that household secondary attack rates (SAR) from asymptomatic cases were only 0.7%. Meanwhile, they found the SAR from symptomatic cases to be significantly higher at 18.0%.(4)
An article from Rational Ground says “the asymptomatic/presymptomatic secondary attack rate is not statistically different from zero, and the confidence interval is technically 0.7 ± 4.2, resulting in a range of -3.5%-4.9%, but attack rates cannot be negative, so it is truncated at 0.”(5)
These findings are groundbreaking, seeing as homes have been categorized as the location with the highest transmission rates.
W.H.O. re-issues the medical device alert
On January 20, 2021, W.H.O. re-issued their medical device alert, reminding testing sites to manually reset their PCR Ct value thresholds to avoid misdiagnoses.
However, they also state that “Where test results do not correspond with the clinical presentation, a new specimen should be taken and retested using the same or different NAT technology.” (6)
Meaning that if the clinical presentation of the patient who tested positive was asymptomatic, then a re-test should be administered with assurance that the PCR Ct value threshold had been manually lowered.
This statement from the re-issued medical device alert makes the connection that those with a higher Ct value will be appear as asymptomatic.
Timing is everything
While the researchers from the University of Florida were able to reveal the unlikelihood of asymptomatic spread, it was the researchers from the Méditerranée Infection University Hospital Institute in Marseille, France, who initially were able to connect Ct values with contagiousness of SARS-CoV-2.
The extensive studies and research show that those who are asymptomatic will test positive with a Ct value > 35, and be non-contagious.
Important questions are beginning to emerge. Were asymptomatic cases non-contagious this whole time? How many false positives were administered? Why wasn’t more research done on Ct values sooner?
The researchers from France urged other testing sites to closely evaluate the Ct value of positive results in April, but there was no genuine acknowledgement until 8 months after the fact.
As we approach the 1-year-marker for when the novel coronavirus was declared a pandemic, ask yourself: how would this past year been different if we viewed SARS-CoV-2 in a different perspective in April, when the French researchers asked us to?
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(1) AACC.org, “SARS-CoV-2 Cycle Threshold: A Metric That Matters (or Not)”, December 3, 2020.
(2) Scola, B. L., Bideau, M. L., Andreani, J., Hoang, V. T., Grimaldier, C., Colson, P., Gautret, P., Raoult, D. “Viral RNA load as determined by cell culture as a management tool for discharge of SARS-CoV-2 patients from infectious disease wards”. European Journal of Clinical Microbiology & Infectious Diseases. 39:1059–1061. https://doi.org/10.1007/s10096-020-03913-9. (2020).
(3) Fauci, Anthony, MD. This Week in Virology. 17 July 2020.
(4) Madewell ZJ, Yang Y, Longini IM, Halloran ME, Dean NE. Household Transmission of SARS-CoV-2: A Systematic Review and Meta-analysis. JAMA Netw Open. 2020;3(12):e2031756. doi:10.1001/jamanetworkopen.2020.31756.
(5) Cabrera, L. “University of Florida researchers find no asymptomatic or presymptomatic spread”. Rational Ground. Retrieved from: https://rationalground.com/university-of-florida-researchers-find-no-asymptomatic-or-presymptomatic-spread/.
(6) World Health Organization. “WHO Information Notice for IVD Users 2020/5: Nucleic acid testing (NAT) technologies that use polymerase chain reaction (PCR) for detection of SARS-CoV-2.” January 20, 2021.