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The following is a curated review of key information and literature about this topic. It is not comprehensive of all data related to this subject.
SARS-CoV-2 is transmitted through “inhalation of virus, deposition of virus on exposed mucous membranes, and touching mucous membranes with soiled hands contaminated with virus” (CDC Science Brief). Respiratory droplets, which are produced when people speak, cough, sneeze and sing, appear to be the predominant mode of transmission. For additional information on routes of transmission of SARS-CoV-2, please see the Personal Protective Equipment in Medical Settings section.
Because most respiratory droplets can travel up to 6 feet, CDC recommends physical distancing, also called “social distancing,” which means keeping a space of at least 6 feet between people who do not live in the same household. Limiting travel outside the home may also be helpful. There may be certain scenarios where distancing alone is inadequate, and therefore a combination of strategies such as masking, hand hygiene and physical distancing is likely most effective.
Physical distancing seeks to limit transmission through “close contact,” which CDC defines as being within 6 feet of an infected person for a cumulative total of 15 minutes or more (e.g., three five-minute exposures for a total of 15 minutes) over a 24-hour period, starting from 2 days before illness onset (or, for asymptomatic patients, 2 days prior to test specimen collection).
Much of the available data on physical distancing to prevent SARS-CoV-2 transmission is limited to observational studies and the extrapolation of data on other respiratory viruses. However, a randomized study in Bangladesh used physician distancing along with other infection prevention measures such as masking and found a reduction in SARS-CoV-2 infection rates (Abaluck, August 2021 – working paper).
In summary: Overall, based on epidemiologic and observational studies, physical distancing, limiting contact outside the household and quarantine are associated with reductions in the transmission of SARS-CoV-2.
Effectiveness of three versus six feet of physical distancing for controlling spread of COVID-19 among primary and secondary students and staff: A retrospective, state-wide cohort study (van den Berg, March 2021).
- 537,336 students and 99,390 staff in Massachusetts.
- Estimated incidence rate ratios for students and staff members in districts with 3 versus 6 feet of physical distancing.
- COVID-19 case rates among students and staff in 251 Massachusetts public school districts that opened for any in-person learning September 2020-January 2021, 194 of which adopted a policy of 6 feet of distancing and 48 of which adopted 3 feet of distancing.
- In total, 537,336 students and 99,390 staff were included in the retrospective cohort study, which included more than 6 million student learning weeks and more than 1 million staff learning weeks.
- During the study period, near-universal masking among students and universal masking among school staff were required by the Massachusetts Department of Elementary and Secondary Education.
- The study found that there was no difference in case rates among either students or staff in districts that adopted a policy of 3 feet versus those that adopted a policy of 6 feet.
- Incident rates among students and staff were both substantially lower than incident rates in the surrounding community.
- Although surveillance testing and contact tracing were not available, if there were a substantial absolute difference in case rates among districts that opted for 3 feet versus 6 feet of distancing, they would have been detected in a dataset this large.
- Limitations include the lack of surveillance testing within the schools.
- The study was conducted in a state with a near universal masking mandate and findings are not generalizable to distancing policies outside of controlled school settings or without masking policies.
Longitudinal pretest-posttest comparison group study to estimate the change in COVID-19 case growth before and after statewide social distancing measures in the United States (Siedner, August 2020).
- An assessment of timing of statewide physical distancing measures and COVID-19 case growth rates.
- Mean daily COVID-19 case growth rate decreased by 0.9% per day beginning 4 days after implementation of the first statewide social distancing measures (P < 0.001).
- Beginning 7 days after physical distancing, the COVID-19 mortality growth rate decreased by 2.0% per day (P < 0.001); this decrease was no longer statistically significant by 10 days.
- This is an observational study and may be limited by confounding factors.
A cohort study examining the association of social distancing, population density, and temperature with SARS-CoV-2 reproduction number over time (Rt) in counties across the United States (Rubin, July 2020).
- Physical distancing was measured by percentage change in visits to nonessential businesses.
- In a multivariable analysis, a 50% decrease in visits to nonessential businesses was associated with a 45% decrease in Rt.
Survey study of people in Maryland regarding the adoption of non-pharmaceutical interventions (Clipman, July 2020).
- 1,030 people were sampled. The median age of the sample was 43 years. 96% had traveled outside of their homes unrelated to work
- 55 participants (5.3%) self-reported testing positive for SARS-CoV-2
- In multivariable analysis, SARS-CoV-2 infection was negatively associated with physical distancing (adjusted odds ratio: 0.10; 95% CI: 0.03 - 0.33)
- This is a survey-based study, and so may be affected by recall bias.
Mathematical modeling study examining the association between mobility patterns and COVID-19 transmission in the United States (Badr, July 2020).
- Cell phone data capturing movement patterns in U.S. counties was used to evaluate how physical distancing affected the rate of new infections.
- Mobility patterns were correlated with decreased COVID-19 growth rates, with a Pearson correlational coefficient of more than 0.7 for 20 of the 25 counties assessed.
- As a modeling study, subject to potential error based on the assumptions used to create the model.
A systematic review and meta-analysis of physical distancing, face masks, and eye protection to transmission of SARS-CoV-2 (Chu, June 2020).
- Review of 172 observational studies in healthcare and non-healthcare settings of > 20,000 patients with COVID-19, MERS, or SARS.
- Maintaining at least 1 meter of physical distance was associated with lower transmission of virus (n=10,736, pooled adjusted odds ratio 0.18, 95% CI 0.09-0.38); protection increased as distance was lengthened (change in relative risk 2.02/m; pinteraction=0.041).
A Cochrane review assessing the effects of quarantine alone or in combination with other measures on COVID-19 control (Nussbaumer-Streit, April 2020).
- Studies reviewing the effect of measures on SARS, MERS, and SARS-CoV-2 were included.
- Modeling studies reported the quarantine of people exposed to confirmed or suspected cases averted 44% to 81% incident cases and 31% to 63% of deaths compared to no measures based on different scenarios (low-certainty evidence).
- When the models combined quarantine with other prevention and control measures (school closures, travel restrictions and physical distancing), they demonstrated a larger effect on the reduction of new cases, transmissions and deaths than individual measures alone (low-certainty evidence).
The impact of social distancing on COVID-19 spread: State of Georgia case study (Keskinocak, October 2020).
An agent-based simulation model to project infection spread in Georgia was developed over 7.5 months, testing different social distancing scenarios, including baselines (no-intervention or school closure only) and combinations of shelter-in-place and voluntary quarantine with different timelines and compliance levels. Compared to peak infections of approximately 180,000 with no intervention and 113,000 with school closure, the model predicted reduced the peak to below 53,000 and delayed it from April to July or later. Increasing shelter-in-place duration from four to five weeks yielded a 2–9% decrease in cumulative infections and a 3–11% decrease in cumulative deaths.
Epidemic mathematical modeling study investigating the effectiveness of social distancing interventions in Seattle, Washington (Matrajt, August 2020).
Interventions reduced contacts of adults >60 years of age, adults 20–59 years of age, and children <19 years of age for 6 weeks. Interventions started earlier in the epidemic delay the epidemic curve and interventions started later flatten the epidemic curve.
Social distancing alters the clinical course of COVID-19 in young adults: A comparative cohort study (Bielecki, June 2020).
Study of 508 soldiers in Switzerland. SARS-CoV-2 infection was examined before and after physical distancing was implemented during an outbreak. 354 soldiers were infected prior to physical distancing, and 30% developed symptoms. 154 soldiers had positive tests after physical distancing; none had symptoms.
Mathematical modeling study seeking to determine the impact of stay-at-home and shelter-in-place orders on COVID-19 transmission in the United States (Fowler, April 2020).
The authors’ model estimated that stay-at-home and shelter-in-place policies reduced COVID-19 infections from a mean incidence reduction of 3.9% one week after implementation (95% CI: 1.2 – 6.6%) to a 22.6% mean reduction 27 days after implementation (95% CI: 14.8 – 30.5%).