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  1. Our take

    In this preprint study which has not yet been peer-reviewed, researchers found that clinical specimens from persons infected with the Delta and Episilon variant had higher infectious viral titers (i.e., amount of replication competent virus) and infectivity (amount of replication competent virus per viral RNA copy) than clinical specimens from persons infected with the Alpha variant. This study provides biological rationale for increased transmission and spread of the Delta variant as compared to other SARS-CoV-2 variants. While this study used rigorous laboratory methods, it was limited by a lack of epidemiological data on specimens, which limits our understanding of how important factors like age or vaccination status may also impact viral RNA levels and infectious viral titer. 

    Study design


    Study population and setting

    This cross-sectional study used 165 clinical specimens (e.g., nasal swabs) identified to be positive for SARS-CoV-2 at the University of Washington Virology Laboratory on March 25, 2021, August 3, 2021, or August 25, 2021. Aliquots of all included specimens were frozen within two days of collection. RT-PCR was used to measure total and subgenomic E RNA levels of SARS-CoV-2. A micro-focus forming assay, which involves growing live virus, was used to measure infectious viral titers of SARS-CoV-2. 

    Summary of main findings

    For each variant, the amount of total and E subgenomic viral RNA levels (i.e., cycle threshold values) were positively correlated with the amount of replication competent virus. Compared to the Alpha variant, the Epsilon and Delta variants had a greater number of infectious units per quantity of total RNA (i.e., infectivity) and subgenomic E RNA. 

    Study strengths

    The study measured infectious viral titers and total and subgenomic viral RNA levels, and compared these values across variants. 


    The major limitation of the study is the lack of clinical and epidemiological data that may impact viral titers (e.g., age, time since symptom onset and vaccination status). Additionally, since not all variants were widely circulating at the same time in the U.S., the included samples were not collected from the same population, nor would sample characteristics that may impact viral titers and culturable virus likely be the same across time when the samples were collected (ex: vaccination status).  

    Value added

    This novel investigation using a micro-focus forming assay demonstrated that the Delta and Epsilon variants of SARS-CoV-2 may be more biologically infectious than the Alpha variant. This information can be useful in understanding how interventions such as masking, ventilation, etc. may need to be increased in response to increased infectivity of specific variants.

  2. SciScore for 10.1101/2021.09.07.21263229: (What is this?)

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

    Table 1: Rigor

    EthicsIRB: The use of deidentified positive specimens for the above studies was approved by the University of Washington Institutional Review Board (STUDY00010205)
    Sex as a biological variablenot detected.
    Randomizationnot detected.
    Blindingnot detected.
    Power Analysisnot detected.
    Cell Line Authenticationnot detected.

    Table 2: Resources

    Cells were permeabilized with 0.1% 100X Triton in 1× PBS for 15 min and then incubated with a primary, cross-reactive rabbit anti-SARS-CoV N monoclonal antibody (Sinobiological, distributed by Thermo Fisher, Cat. #40143-R001 at a dilution of 1:20,000) followed by a peroxidase-labelled goat anti-rabbit antibody (SeraCare, Milford, MA, USA, Cat. #5220-0336 diluted to 1:4,000) and then the peroxidase substrate (SeraCare, Cat. #5510-0030).
    anti-SARS-CoV N
    suggested: None
    suggested: (SeraCare KPL Cat# 5220-0336, RRID:AB_2857917)
    Experimental Models: Cell Lines
    Serial ten-fold dilutions of clinical sample were used to inoculate Vero E6-TMPRSS2 cell monolayers (60,000 cells/well seeded one day prior) in 96□well white polystyrene microplates (Falcon, Cat.
    Vero E6-TMPRSS2
    suggested: None
    Recombinant DNA
    RT-PCR reactions used one of two sets of primers/probes: 1) WHO-E, using the E_Sarbeco-F/R/P set34; 2) Mills-sgE, using sgLeader-F2, sgE-R, and sgE-P23.
    suggested: None

    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.

    Results from scite Reference Check: We found no unreliable references.

    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.