Review Article

Physical distancing for reducing and mitigating COVID-19 transmission: A rapid review

Ekpereonne B. Esu, Ehimario U. Igumbor, Anthony T. Okoro, Patience O. Idiege, Kelechi Meremikwu, Angela E. Oyo-Ita, Kathleen A. Dunn, Hannah Hamilton-Hurwitz, Martin M. Meremikwu

Received: 10 May 2024; Accepted: 07 Sept. 2024; Published: 06 Mar. 2025

Copyright: © 2025. The Author(s). Licensee: AOSIS.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Background: Physical distancing is an infection prevention and control (IPC) intervention and public health and social measure (PHSM) aimed at reducing respiratory infections. However, the optimal distance for preventing severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) transmission remains uncertain.

Aim: This study aimed to determine whether a distance of 1 m is optimal for reducing SARS-CoV-2 transmission.

Setting: Community and healthcare settings.

Methods: The Central Register of Controlled Trials, Cochrane Database of Systematic Reviews, PubMed and Embase were searched for studies conducted in community, healthcare and other defined settings between 01 January 2020 and 07 September 2022. The quality of the evidence was assessed with the ROBINS-I tool. This study is registered on PROSPERO (CRD42022355706).

Results: One retrospective cohort study enrolled 242 school districts in Massachusetts, United States, which included 537 336 students and 99 390 staff attending in-person instruction between September 2020 and January 2021. No significant differences in SARS-CoV-2 cases were observed among both groups for students (adjusted incidence rate ratio [aIRR]: 0.904; 95% confidence interval [CI]: 0.616–1.325) and staff (aIRR: 1.015; 95% CI: 0.754–1.366).

Conclusion: In this sole study, SARS-CoV-2 cases were similar regardless of physical distance (1 metre vs 2 metres), although they studied a limited number of events. Physical distancing was part of a broader set of mitigation measures employed in the study setting; including consistent mask use.

Contribution: The effectiveness of physical distancing against SARS-CoV-2 infection is significantly influenced by mask use, thus its impact on physical distancing effectiveness cannot be separated. Further studies are needed to increase the certainty of the evidence.

Keywords: physical distancing; SARS-CoV-2; COVID-19; systematic review; rapid review.

Introduction

COVID-19 pandemic caused by the rapid and global spread of a novel coronavirus named severe acute respiratory coronavirus 2 (SARS-CoV-2), has had a profound effect on global health and the world economy.^1,2 As of 04 August 2024, more than 775 million cases of COVID-19 have been reported to the World Health Organization (WHO), including more than 7 million deaths.³

COVID-19 patients have reported symptoms such as fever, cough, general weakness and/or fatigue, headache, myalgia, sore throat, coryza, dyspnoea, nausea, diarrhoea and anorexia.^4,5,6 The risk of transmission is associated with proximity to an infectious person, and the risk is higher in crowded, poorly ventilated areas, especially in indoor spaces.^7,8

Public health agencies have recommended infection prevention and control (IPC) intervention and public health and social measures (PHSM) to prevent the spread of SARS-CoV-2 in healthcare and community settings. These include wearing face masks, keeping a physical distance, regularly cleaning hands with soap and water or the use of alcohol-based hand rub, practicing respiratory etiquette and ensuring proper ventilation.⁹ Consistent application of IPC and PHSM measures is essential to preventing transmission of SARS-CoV-2 in healthcare and community settings. No single intervention provides complete protection; instead, measures are used in combination with one another to achieve effective prevention.^10,11,12 Physical distancing has been integral to IPC and PHSM measures in the context of COVID-19; however, there remains no consensus on what precise physical distance between individuals would optimally reduce the likelihood of SARS-CoV-2 transmission. This review assessed whether a distance of one metre is optimal for reducing and mitigating the transmission of SARS-CoV-2. The WHO used this review to inform evidence-based recommendations on physical distancing.

Methods

Protocol and registration

The systematic review protocol was prospectively published with the International Prospective Register of Systematic Review (PROSPERO) at the Centre for Reviews and Dissemination, University of York (CRD42022355706). The final review was reported following the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) guidelines.¹³

Eligibility criteria

The criteria for considering studies for inclusion in the systematic review were as follows:

• Study design: Randomised controlled trials and other comparative study designs, including cohort studies (prospective or retrospective), case control studies, controlled before-and-after studies and interrupted time series, were considered.
• Population: Populations of interest included all human populations in the community and healthcare settings.
• Intervention: The primary focus was on studies that considered physical distancing when people were co-located in the same physical space, keeping a distance of one metre compared to when people were co-located in the same physical space, keeping less than or more than one metre. Studies that allocated an additional intervention to both the intervention and control group were eligible to be included, provided the other intervention was the same for each group.
• Outcome: Outcome measure was SARS-CoV-2 infection in humans.

Information sources

The following databases were searched: The Cochrane Library – Central Register of Controlled Trials (CENTRAL) and Cochrane Database of Systematic Reviews, PubMed and Embase for the period 2020 to 07 September 2022 for potentially relevant studies. A ‘snowball’ search was used to identify additional studies by searching the reference lists of retrieved full-text studies. No language restrictions were applied in the search. The reporting of the search and selection of studies was in accordance with PRISMA guidelines. All citations identified from the database searches were uploaded to DistillerSR,¹⁴ an online software for managing systematic reviews for study selection.

Search strategy

For this review, the search strategy was tailored to each database using index terms, medical subject headings, truncations and Boolean operators. In addition, a combination of terms describing the key concepts of physical distancing, SARS-CoV-2 and study design filters were used. The complete line-by-line search strategy can be found in Appendix 1 Figure 1-A1.

Study selection

Four review authors in pairs (E.B.U., A.T.O., P.O.I., K.M.) independently screened the titles and abstracts of the deduplicated search results for relevant studies. Disagreements were resolved through consensus within the review team. Next, two review authors (E.B.U., A.T.O.) independently screened full-text articles for inclusion. Again, disagreements were resolved through consensus within the review team. Reviewers independently applied the inclusion criteria to the full-text reports using an eligibility form. All publications were scrutinised to ensure each study was included in the review only once. A PRISMA flow diagram summarises the search and selection of studies (Figure 1).

FIGURE 1: Identification of studies flow diagram.

Data extraction

Two authors used a data extraction form (E.B.U., A.T.O.) to extract data from the eligible study. The extracted data were compared and discrepancies were resolved through discussion. Critical data extracted from the study included the study reference, objective, setting, sample for intervention and control groups, participants’ characteristics, intervention description and outcomes. Data were extracted on any measure of SARS-CoV-2 infection.

Risk of bias assessment

One review author (E.B.U.) assessed the risk of bias in the included study using the risk of bias in non-randomised studies of interventions (ROBINS-I) tool.¹⁵ The assessment was cross-checked by other review authors (A.T.O., P.O.I., K.M.). Judgements of ‘yes’, ‘no’ and ‘unclear’ indicated a low, high or unclear risk of bias, respectively.

Assessment of the certainty of evidence

Certainty of the evidence was ranked using the Grading of Recommendations, Assessment, Development and Evaluations (GRADE) guidelines for no single or no pooled estimate of effect.¹⁶ Ratings were done using the GRADEpro Guideline Development Tool (GDT) (https://gdt.gradepro.org/app/#organizations). The quality of evidence was appraised for each outcome against five criteria: risk of bias (an appraisal of the overall risk of bias for trials contributing to the outcome); consistency (an evaluation of explained and unexplained heterogeneity); directness (an appraisal of how directly the included trials address the review question); precision (an assessment of the statistical precision of the result); and publication bias (an assessment of the risk of publication bias).

Synthesis methods

Should meta-analysis have been possible, data would have been pooled using risk or rate ratios with 95% confidence intervals (CIs) and a random-effects model. We planned to present results for continuous outcomes as mean differences (MD) and time-to-event outcomes as hazard ratios, all with 95% CIs. Given that there was only one study for inclusion, a qualitative synthesis was conducted according to the synthesis without meta-analysis (SWiM) guidelines.¹⁷

Assessment of reporting biases

Funnel plots for meta-analyses that included at least 10 studies to assess small-study effects were planned. However, as there was only one included study, this analysis was not feasible.

Results

Figure 1 shows the study selection process. The search of electronic databases retrieved 1879 records, with seven additional records identified through searching reference lists of potentially relevant studies. After removing duplicates, 1862 records were screened based on title and abstract, after which 91 were selected for full-text screening. Full-text screening excluded 89 records because they were not comparative studies or did not report interventions of interest. Only one study¹⁸ was included in this systematic review.

Characteristics of the included study

One retrospective cohort study¹⁸ met the inclusion criteria for this review (Table 1). It was conducted across 242 school districts in Massachusetts, United States, between September 2020 and January 2021. The study enrolled 537 336 kindergarten through 12th-grade students and 99 390 staff participating in in-person learning during the COVID-19 pandemic. The intervention group comprised of school districts implementing at least 3 feet (1 metre) of social distancing, and the control group was comprised of school districts implementing at least 6 feet (2 metres) of social distancing. In addition to physical distancing, intervention and control groups implemented universal masking and influenza vaccination.

Results of the included study

There was no significant difference in SARS-CoV-2 cases between both groups (school districts implementing at least 3 ft of physical distancing versus at least 6 ft). Student case rates were similar in the 242 districts with ≥ 3 versus ≥ 6 ft of physical distancing between students after adjustment for community incidence (adjusted incident rate ratio [IRR]: 0.904; 95% confidence interval [CI]: 0.616–1.325). Cases among school staff in districts with ≥ 3 ft versus ≥ 6 ft of physical distancing were also similar (adjusted IRR: 1.015; 95% CI: 0.754–1.366).

Risk of bias

According to the ROBINS-I tool, the risk of bias was rated overall as moderate. The study was at low risk of bias for six of the seven domains. Thus, the domain ‘bias due to deviations from intended intervention’ was rated as moderate risk as the study authors did not measure adherence to physical distancing. Furthermore, the environment in which this study was performed had concurrent policies for universal masking and influenza vaccination, thus potentially affecting the outcome (see Table 2).

Certainty of evidence

The certainty of evidence for the only outcome of the review, SARS-CoV-2 infection, was very low (Table 3). The certainty of evidence was downgraded by one because of serious concerns with study design limitations. We also downgraded the certainty by one on account of imprecision, as the 95% CI for the outcome included substantial benefit and harm.

Discussion

This systematic review summarised all the evidence on whether a physical distance of one metre was optimal for reducing and mitigating the transmission of SARS-CoV-2 in the community, healthcare settings or other defined settings. Although several studies^{19,20,21,22,23,24,25,26,27,28,29,30} have evaluated the effectiveness of physical distancing as one of the core IPC measures aimed at reducing the spread of SARS-CoV-2, there is a lack of research evidence on the comparative effectiveness of various distances. The certainty of the evidence is very low because of design limitations and imprecision of the estimates.

Comparison with other systematic reviews

There have been other systematic reviews that have evaluated the effectiveness of physical distancing, though none compared various distances of separation. One additional systematic review by Chu and colleagues concluded that a physical distance of one metre is associated with a large reduction in infection and that distances of two metres might be more effective, though this was largely based on modelling and indirect evidence.³¹ Another systematic review assessing the effectiveness of public health measures in reducing the incidence of COVID-19 (5 studies with a total of 2727 people with SARS-CoV-2 and 108 933 participants) found physical distancing to be associated with a 25% reduction in the incidence of COVID-19 (relative risk 0.75, 95% CI: 0.59 to 0.95, I² = 87%).³² In that systematic review, assessing the effect of different distance recommendations for physical distancing was not feasible.

The other systematic reviews identified,^{33,34,35,36,37} summarised the evidence from observational studies that studied the effectiveness of physical distancing but did not compare different distances.

Strengths and limitations

The review used a comprehensive search strategy to identify and select studies without language restrictions. In addition, this review followed the robust and rigorous systematic review methodology developed and recommended by Cochrane.³⁸ A limitation of the review is the lack of comparative studies. As a result, the conclusions drawn from the current best evidence may need to be revised as more studies become available. The sole study included in this review did not assess adherence to physical distancing. Since the effectiveness of physical distancing and other PHSM and IPC interventions heavily relies on compliance, future research should focus on monitoring adherence to physical distancing interventions. Moreover, additional comparative studies or at least natural experimental studies are needed to better understand the optimal physical distance for mitigating the transmission of SARS-CoV-2. Physical distancing was just one element of a broader set of mitigation measures employed in the study setting which included universal masking and influenza vaccination. Given the outcome of interest was SARS-CoV-2 transmission, seasonal influenza vaccination is unlikely to affect the outcome.³⁹ The concurrent use of multiple potentially effective interventions in this study made it difficult to isolate the effect of physical distancing from the effect of the other intervention on SARS-CoV-2 transmission.⁴⁰

Conclusion

One study addressed the PICO question, and very low certainty evidence indicates that SARS-CoV-2 cases were similar irrespective of the physical distance (1 metre vs 2 metres) between participants. Importantly, this study had estimates that were very imprecise because of a limited number of events. Relying on a single study is insufficient and does not provide a robust basis for drawing definitive conclusions, especially when paired with universal mask use. Further research is needed to increase the certainty of the evidence regarding the effectiveness of a physical distance of one metre compared to distances greater or less than one metre in reducing and mitigating the transmission of SARS-CoV-2.

Acknowledgements

The authors thank their colleagues at Infection Prevention and Control, Country Readiness Strengthening, World Health Organization, World Health Emergencies Programme, Geneva, Switzerland, for their support during the preparation of the review.

The authors declare that they have no financial or personal relationships that may have inappropriately influenced them in writing this article.

H.H.-H., K.A.D. and M.M.M. conceptualised the review. E.B.U., A.T.O., P.O.I. and K.M. were involved with data curation and formal analysis. E.B.U., E.U.I., A.E.O.-I., K.A.D., H.H.-H. and M.M.M. wrote the original draft of the article.All the authors reviewed and edited the article draft and approved the publication of the final version.

Ethical approval was not required as the work is based on published scholarly documents in the public domain.

The review was funded by a grant from Country Readiness Strengthening, WHO World Health Emergencies Programme, Geneva, Switzerland to Cochrane Nigeria.

Data sharing is not applicable to this article as no new data were created or analysed in this study.

The views and opinions expressed in this article are those of the authors and are the product of professional research. The article does not necessarily reflect the official policy or position of any affiliated institution, funder, agency or that of the publisher. The authors are responsible for this article’s results, findings and content.

References

Appendix 1

FIGURE 1-A1: Search strategy.

FIGURE 1-A1 (Continues…): Search strategy.

FIGURE 1-A1 (Continues…): Search strategy.