Researchers analysed around 72,500 human saliva samples for SARS-CoV-2 at the University of Colorado, Boulder and found that asymptomatic individuals on the college campus carried just as much virus as individuals hospitalised with COVID-19.

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The study suggested that, at any given time, approximately 2 per cent of individuals carried 90 per cent of the SARS-CoV-2 virions circulating within the community and acted as super-carriers and, possibly, super-spreaders, according to the authors. The study, published in Proceedings of the National Academy of Sciences of the United States of America (PNAS), found that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel coronavirus that emerged into the human population in late 2019, presumably from animal reservoirs. During the ensuing worldwide pandemic, already more than 3 million lives have been lost due to the virus. The spread of SARS-CoV-2 has thus far been extremely difficult to contain.

One key reason for this is that both presymptomatic and asymptomatic infected individuals can transmit the virus to others. Further, it is becoming clear that certain individuals play a key role in seeding superspreading events. "Viral loads were measured in saliva, which has proven to be an accessible and reliable biospecimen in which to identify carriers of this respiratory pathogen, and the most likely medium for SARS-CoV-2 transmission" said researchers.

"Our dataset is unique in that all SARS-CoV-2-positive individuals reported no symptoms at the time of saliva collection, and therefore were infected but asymptomatic or presymptomatic. We find that the distribution of SARS-CoV-2 viral loads on our campus is indistinguishable from what has previously been observed in symptomatic and hospitalised individuals. Strikingly, these datasets demonstrate dramatic differences in viral levels between individuals, with a very small minority of the infected individuals harbouring the vast majority of the infectious virions", they said.
During the fall 2020 semester, more than 72,500 saliva samples were screened for SARS-CoV-2. A qRT-PCR assay was used, with the template coming from the direct addition of saliva without RNA purification (22). Three TaqMan primer/probe sets were used in a multiplex reaction directed against two regions of the SARS-CoV-2 genome (CU-E and CU-N, where CU stands for the University of Colorado) and a host transcript (CU-RNaseP) as control.

The multiplex reaction was used to create standard curves to convert the Ct value (cycle threshold) of each primer set to viral load (virions per millilitre) in the original saliva sample. To ensure the viral load quantification is accurate for samples with extremely low Ct values (i.e., extremely high viral loads), the researchers performed serial dilution of three saliva samples with among the highest observed viral loads of the semester and showed that Ct values scale linearly with the dilution factor.

From over 72,500 saliva samples screened, 1,405 SARS-CoV-2-positive samples were identified. The vast majority of these positive samples were from unique individuals because individuals with positive tests were directed into the health care system for further testing and care. The distribution of the Ct values of these 1,405 individuals, with each of the two primer, sets used. Overall, the distribution of SARS-CoV-2 viral load fits under a log-normal distribution centered around the mean of 2.1 x 107 virions per mL (median = 1.1 x 106 virions per mL) for the CU-E primers or 5.9 x 106 virions per mL (median = 2.5 x 105 virions per mL) for the CU-N primers.

The highest observed viral load was over 6 trillion (6.1 x 1012) virions per mL, which was only observed in one individual. It is remarkable to consider that this individual was on campus and reported no symptoms at the testing site. The lowest viral load detected was eight virions per millilitre. Thus, surveillance testing demonstrates an extremely wide variation in the viral load in infected but seemingly healthy (asymptomatic) individuals.