Higher viral load drives infrequent SARS-CoV-2 transmission between asymptomatic residence hall roommates

This article has been Reviewed by the following groups

Read the full article See related articles


In 2019-2020, the COVID-19 pandemic spread to over 200 countries in less than six months. To understand the basis of this aggressive spread, it is essential to determine the transmission rate and define the factors that increase the risk of transmission. One complication is the large fraction of asymptomatic cases, particularly in young populations: these individuals have viral loads indistinguishable from symptomatic people and do transmit the SARS-CoV-2 virus, but they often go undetected. As university students living in residence halls commonly share a small living space with roommates, some schools established regular, high density testing programs to mitigate on-campus spread. In this study, we analyzed longitudinal testing data of residence hall students at the University of Colorado Boulder. We observed that students in single rooms were infected at a lower rate than students in multiple occupancy rooms. However, this was not due to high rates of transmission between roommates, which only occurred approximately 20% of the time. Since these cases were usually asymptomatic at the time of diagnosis, this provides further evidence for asymptomatic transmission. Notably, individuals who likely transmitted to their roommates had an average viral load ∼6.5 times higher than individuals who did not. Although students were moved to separate isolation rooms after diagnosis, there was no difference in time to isolation between these cases with or without transmission. This analysis argues that inter-roommate transmission occurs in a minority of cases in university residence halls and provides strong correlative evidence that viral load can be proportional to the probability of transmission.

Article activity feed

  1. Our take

    This study, available as a preprint and thus not yet peer-reviewed, examined the role of living in multiple (vs. single) occupancy rooms, viral load, and time to isolation following COVID-19 diagnosis on transmission of SARS-CoV-2 among student roommates at a university in Colorado during the Fall 2020 semester. Students provided proof of negative SARS-CoV-2 test prior to move-in to residential halls and underwent weekly RT-qPCR screening. Overall, 1058 of 6408 residential students (16.5%) were diagnosed with COVID-19 during the semester. Students in multiple occupancy rooms were almost twice as likely to be infected with COVID-19 compared to those in single occupancy rooms (19.1% vs. 10.3%); viral load was positively associated with higher probability of transmission, but time to isolation was not. Mandatory weekly PCR based testing was a strength of this study, and gave confidence about coverage of incident infections among students. However, there was no information on the number of students who had antibodies prior to campus arrival, and findings may not reflect the infectiousness and virulence of SARS-CoV-2 variants that have become more common in the US in 2021.

    Study design


    Study population and setting

    This study investigated factors associated with transmission of SARS-CoV-2 among student roommates at the University of Colorado, Boulder, USA, during the Fall 2020 academic semester (August 17 to November 25). Specifically, the authors examined the extent of transmission among roommates, and the role that viral load and time spent by an infected individual in the shared room (before moving to isolation), played in the probability of transmission. All students had to have proof of negative RT-qPCR test taken less than five days before move-in to the residential halls. Furthermore, all asymptomatic residential students underwent compulsory weekly RT-qPCR screening, which was followed by confirmatory diagnostic RT-qPCR testing among those infected. Confirmation of COVID-19 was followed by isolation for 10 days, either on-campus or at the student’s permanent off-campus home. Students with a diagnosis of COVID-19 were exempted from weekly tests for the rest of the semester. Among students in multiple occupancy rooms, likely transmission was defined as roommates detected within 14 days of each other, while roommates detected on the same day were considered to have been infected by the same external sources but not roommate transmission. Roommates detected > 14 days were considered to have been infected by independent, external sources. Multiple occupancy rooms with only one case were not considered to be roommate transmission. Viral load was assessed by quantification of the viral E gene RNA in the RT-qPCR screening test.

    Summary of main findings

    In total, 16.5% residential students (1058/6408) were diagnosed with COVID-19. Students in multiple occupancy rooms were almost twice as likely to be infected with COVID-19 compared to students in single occupancy rooms (19.1% vs. 10.3%) even though test frequency was not significantly different between the groups. Among 574 multiple occupancy rooms where at least one student was positive, and there was adequate information, 7.7% had multiple positive cases detected concurrently, 20.2% had likely in-room transmission, and 72.1% did not appear to have in-room transmission. The authors noted that most transmission appeared to have originated with off-campus contacts based on case investigation and contact tracing, with possible on-campus transmission, especially among students living together. Rooms with likely in-room transmission had a 6.5-fold greater average viral load than rooms that did not appear to have transmission events. Time to isolation was not associated with increased transmission, even though 18.6% of unlikely transmission cases entered isolation on the day of COVID-19 detection compared to 9.6% of transmission cases.

    Study strengths

    Proof of negative RT-qPCR before move-in, as well as mandatory weekly RT-qPCR screening of all asymptomatic students living in residence halls, which was followed by confirmatory diagnostic RT-qPCR testing among those infected are strengths of this study. Weekly tests meant that most cases were identified early, thus reducing possible bias of sampling on viral load. Authors undertook robust analytical steps to determine likely sources of transmission among students in multiple occupancy rooms and provided evidence that test frequency bias between students in single vs multiple occupancy rooms was unlikely to explain the results.


    The study was conducted before widespread circulation of the mutated variants of SARS-CoV-2 (that originated in UK, South Africa, and Brazil), within the United States. Thus, findings may not reflect the infectiousness and virulence of these variants. Some students did not have enough information on the testing record which led to potential bias, as they were excluded from some analyses. There was no determination of prevalence of SARS-CoV-2 antibodies among students prior to arrival on campus.

    Value added

    This analysis provides evidence of asymptomatic transmission of SARS-CoV-2 among student roommates, and evidence that higher viral load is associated with greater transmission.

  2. SciScore for 10.1101/2021.03.09.21253147: (What is this?)

    Please note, not all rigor criteria are appropriate for all manuscripts.

    Table 1: Rigor

    Institutional Review Board Statementnot detected.
    Randomizationnot detected.
    Blindingnot detected.
    Power Analysisnot detected.
    Sex as a biological variablenot detected.

    Table 2: Resources

    No key resources detected.

    Results from OddPub: We did not detect open data. We also did not detect open code. Researchers are encouraged to share open data when possible (see Nature blog).

    Results from LimitationRecognizer: An explicit section about the limitations of the techniques employed in this study was not found. We encourage authors to address study limitations.

    Results from TrialIdentifier: No clinical trial numbers were referenced.

    Results from Barzooka: We did not find any issues relating to the usage of bar graphs.

    Results from JetFighter: We did not find any issues relating to colormaps.

    Results from rtransparent:
    • Thank you for including a conflict of interest statement. Authors are encouraged to include this statement when submitting to a journal.
    • Thank you for including a funding statement. Authors are encouraged to include this statement when submitting to a journal.
    • No protocol registration statement was detected.

    About SciScore

    SciScore is an automated tool that is designed to assist expert reviewers by finding and presenting formulaic information scattered throughout a paper in a standard, easy to digest format. SciScore checks for the presence and correctness of RRIDs (research resource identifiers), and for rigor criteria such as sex and investigator blinding. For details on the theoretical underpinning of rigor criteria and the tools shown here, including references cited, please follow this link.