Long distance airborne transmission of SARS-CoV-2: rapid systematic review

  1. Daphne Duval
  2. Jennifer C Palmer
  3. Isobel Tudge
  4. Nicola Pearce-Smith
  5. Emer O’Connell
  6. Allan Bennett
  7. Rachel Clark

Reviewed by

Read the full article See related articles

Listed in

Log in to save this article

Abstract

Objectives

To evaluate the potential for long distance airborne transmission of SARS-CoV-2 in indoor community settings and to investigate factors that might influence transmission.

Design

Rapid systematic review and narrative synthesis.

Data sources

Medline, Embase, medRxiv, Arxiv, and WHO COVID-19 Research Database for studies published from 27 July 2020 to 19 January 2022; existing relevant rapid systematic review for studies published from 1 January 2020 to 27 July 2020; and citation analysis in Web of Science and Cocites.

Eligibility criteria for study selection

Observational studies reporting on transmission events in indoor community (non-healthcare) settings in which long distance airborne transmission of SARS-CoV-2 was the most likely route. Studies such as those of household transmission where the main transmission route was likely to be close contact or fomite transmission were excluded.

Data extraction and synthesis

Data extraction was done by one reviewer and independently checked by a second reviewer. Primary outcomes were SARS-CoV-2 infections through long distance airborne transmission (>2 m) and any modifying factors. Methodological quality of included studies was rated using the quality criteria checklist, and certainty of primary outcomes was determined using the Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) framework. Narrative synthesis was themed by setting.

Results

22 reports relating to 18 studies were identified (methodological quality was high in three, medium in five, and low in 10); all the studies were outbreak investigations. Long distance airborne transmission was likely to have occurred for some or all transmission events in 16 studies and was unclear in two studies (GRADE: very low certainty). In the 16 studies, one or more factors plausibly increased the likelihood of long distance airborne transmission, particularly insufficient air replacement (very low certainty), directional air flow (very low certainty), and activities associated with increased emission of aerosols, such as singing or speaking loudly (very low certainty). In 13 studies, the primary cases were reported as being asymptomatic, presymptomatic, or around symptom onset at the time of transmission. Although some of the included studies were well conducted outbreak investigations, they remain at risk of bias owing to study design and do not always provide the level of detail needed to fully assess transmission routes.

Conclusion

This rapid systematic review found evidence suggesting that long distance airborne transmission of SARS-CoV-2 might occur in indoor settings such as restaurants, workplaces, and venues for choirs, and identified factors such as insufficient air replacement that probably contributed to transmission. These results strengthen the need for mitigation measures in indoor settings, particularly the use of adequate ventilation.

Systematic review registration

PROSPERO CRD42021236762.

Article activity feed

  1. Version published to 10.1136/bmj-2021-068743
  2. ScreenIT

    SciScore for 10.1101/2021.10.19.21265208: (What is this?)

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

    Table 1: Rigor

    Ethicsnot detected.
    Sex as a biological variablenot detected.
    Randomizationnot detected.
    Blindingnot detected.
    Power Analysisnot detected.

    Table 2: Resources

    Software and Algorithms
    SentencesResources
    For search dates from 27 July 2020 to 21 April 2021, electronic searches were conducted in MEDLINE, Embase, medRxiv, Arxiv and WHO COVID-19 Research Database (initial search conducted on the 8 February 2021 and updated on the 21 April 2021).
    MEDLINE
    suggested: (MEDLINE, RRID:SCR_002185)
    Embase
    suggested: (EMBASE, RRID:SCR_001650)

    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: We detected the following sentences addressing limitations in the study:
    Strengths and limitations: This rapid systematic review is the first to critically assess the likelihood of long-distance airborne transmission of SARS-CoV-2 using only direct real-world evidence from observational studies in indoor community settings. The risk associated with close contact transmission is now relatively well-known, at least in indoor settings, but there remains a need to further understand long-distance transmission to support public health decision making, especially in relation to the implementation of effective and proportionate mitigation measures in indoor community settings. Another strength of this review is the application of inclusion criteria that focused the critical appraisal on those studies which involved comprehensive epidemiological investigations. Some of these studies did not only include epidemiological data, but also genomic analysis, video surveillance, analysis of seating arrangements and environmental hypothesis testing. This has enabled this synthesis to provide the best evidence currently available to examine the potential for long-range airborne transmission in real-world settings. The main limitation of selecting studies of only real-world human-to-human transmission events is that scenarios where transmission has not occurred will not be reported, and likewise where transmission events have not been detected by contact tracing systems - this could be seen as a form of publication bias. All of the evidence is from retrospective epi...

    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.

  3. Version published to 10.1101/2021.10.19.21265208 on medRxiv