May 7, 2021

Just 2% of SARS CoV-2 positive carry 90% of the virus

Just 2% of SARS-CoV-2-positive individuals carry 90% of the virus circulating in communities

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Just 2% of SARS CoV-2 positive individuals carry 90% of the virus circulating in communities


We analyze data from the Fall 2020 pandemic response efforts at the University of Colorado Boulder (USA), where more than 72,500 saliva samples were tested for SARS CoV-2 using quantitative RT-PCR. All samples were collected from individuals who reported no symptoms associated with COVID-19 on the day of collection. From these, 1,405 positive cases were identified.

The distribution of viral loads within these asymptomatic individuals was indistinguishable from what has been previously reported in symptomatic individuals. Regardless of symptomatic status, approximately 50% of individuals who test positive for SARS-CoV-2 seem to be in non-infectious phases of the disease, based on having low viral loads in a range from which live virus has rarely been isolated. We find that, at any given time, just 2% of individuals carry 90% of the virions circulating within communities, serving as viral super-carriers and possibly also super-spreaders.

Competing Interest Statement

Some of the authors of this study have financial ties to companies that offer commercial SARS CoV-2 testing (NRM, QY, CLP, SLS are co-founders of Darwin Biosciences; TKS, EL, and PKG are co-founders of TUMI Genomics). RP is a co-founder of Faze Medicines and RDD is a co-founder of Arpeggio Biosciences.

Funding Statement

This work was supported by funding from the United States CARES Act (to the University of Colorado), the Burroughs Wellcome Fund (PATH award to SLS; PDEP award to NRM), the National Institutes of Health (DP1-DA-046108 to SLS), the Interdisciplinary Quantitative Biology Program (to SSW) and Howard Hughes Medical Institute (to RP).

Author Declarations

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.


The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

Viral load data on student participants were deidentified and aggregated from the University of Colorado Boulder operational screening for SARS CoV-2. This activity does not meet the definition of human subject research described in the United States Health and Human Services 45 Code of Federal Regulations Part 46. Deidentified saliva samples (n=105) used for the cross-comparison of primers were collected under protocol 20-0662, approved by the University of Colorado Boulder Institutional Review Board.

All necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived.


I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).


I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable.

Paper in collection COVID-19 SARS CoV-2 preprints from medRxiv and bioRxiv

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