What is the evidence for staying two meters away from each other during social distancing?

Posted on April 17, 2020 by Madalon Burnett

Summary

  • The WHO recommends a distance of 1m for social distancing and the CDC recommends a distance of 2m for social distancing. 
  • We know that COVID-19 can be transmitted through droplets, though it does not appear to travel airborne. We do not know what size of droplets carry COVID-19 or how long the virus remains viable within the droplet. 
  • Droplets of various sizes can definitely travel 2m, and so being within 2m of a carrier of the virus puts you at risk of droplet transmission. Droplets may be able to travel farther, and so we do not know the distance that makes you 100% safe from droplet transmission.

 

The following guidelines and information do NOT include situations with aerosol generating procedures. 

I.   CURRENT SOCIAL DISTANCING RECOMMENDATIONS

  • The WHO recommends maintaining 1m between yourself and anyone who is coughing or sneezing (1)
  • The CDC recommends that you stay at least 6 feet (2 meters) from other people to practice social distancing effectively (2)
  • There is some evidence to suggest that these recommendations may underestimate the distance and time scale that the virus can travel in droplets generated by sneezing (3)

II.   MECHANISM OF TRANSMISSION OF COVID-19

  • Case study of three patients in airborne infection isolation rooms (4). One patient showed viral shedding in these places:
    • Extensive contamination of the room including bed, light switches, chair, stethoscope, and glass door (13 of 15 room sites tested positive)
    • Air outlet fan tested positive, suggesting droplets can carry virus through airflow
    • Toilet bowl and sink samples were positive, suggesting fecal shedding could be a source of transmission
  • By convention, diseases with respiratory symptoms are thought to have droplet transmission, but there is evidence that this virus is transmitted in the stool as well (5)
  • In studies that collected air samples, there was no virus detected, suggesting that the virus is not typically airborne (6, 4). This is controversial and needs further study (7)

 

Based on the above, the most likely route of transmission for COVID-19 is droplet transmission, and thus the recommendations from the WHO and CDC are based on droplet precautions.

III.   DROPLET PRECAUTIONS

  • According to the WHO (8)
    • “Respiratory infections can be transmitted through droplets of different sizes: when the droplet particles are >5-10 μm in diameter they are referred to as respiratory droplets, and when then are <5μm in diameter, they are referred to as droplet nuclei”
    • “Droplet transmission occurs when a person is in in close contact (within 1 m) with someone who has respiratory symptoms (e.g., coughing or sneezing) and is therefore at risk of having his/her mucosae (mouth and nose) or conjunctiva (eyes) exposed to potentially infective respiratory droplets. Transmission may also occur through fomites in the immediate environment around the infected person.”
  • Droplets  80 microns in diameter or larger do not typically travel in the air farther than 2m in a controlled environment, but smaller droplets can (9)
  • The distance that droplets can transmit virus depends on the size of the droplet, the humidity of the air, and expiratory action they come from (coughing vs sneezing vs laughing vs talking). These factors influence the distance travelled by the droplet and the viability of the virus upon arrival (10)
  • Droplet dynamics are poorly understood, although mathematical models show that droplets of a variety of sizes can definitely travel within 2m (11)

Questions? Comments? Does this need to be updated? Do you have valuable points to add ? Please email ask.reakt@ubc.ca.

References

  1. Advice for public [Internet]. Who.int. 2018 [cited 2020 Apr 17]. Available from: https://www.who.int/emergencies/diseases/novel-coronavirus-2019/advice-for-public
  2. CDC. Social Distancing, Quarantine, and Isolation [Internet]. Centers for Disease Control and Prevention. 2020 [cited 2020 Apr 17]. Available from: https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/social-distancing.html
  3. Bourouiba L. Turbulent Gas Clouds and Respiratory Pathogen Emissions. JAMA [Internet]. 2020 Mar 26 [cited 2020 Apr 17]; Available from: https://jamanetwork.com/journals/jama/fullarticle/2763852
  4. Ong SWX, Tan YK, Chia PY, Lee TH, Ng OT, Wong MSY, et al. Air, Surface Environmental, and Personal Protective Equipment Contamination by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) From a Symptomatic Patient. JAMA [Internet]. 2020 Mar 4 [cited 2020 Apr 17]; Available from: https://jamanetwork.com/journals/jama/fullarticle/2762692
  5. Hindson J. COVID-19: faecal–oral transmission? Nature Reviews Gastroenterology & Hepatology [Internet]. 2020 Mar 25 [cited 2020 Apr 18]; Available from: https://www.nature.com/articles/s41575-020-0295-7
  6. Cheng VCC, Wong S-C, Chen JHK, Yip CCY, Chuang VWM, Tsang OTY, et al. Escalating infection control response to the rapidly evolving epidemiology of the coronavirus disease 2019 (COVID-19) due to SARS-CoV-2 in Hong Kong. Infection Control & Hospital Epidemiology [Internet]. 2020 Mar 5 [cited 2020 Apr 18];1–6. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7137535/
  7. The coronavirus pandemic and aerosols: Does COVID-19 transmit via expiratory particles? [Internet]. Aerosol Science and Technology. 2020 [cited 2020 Apr 18]. Available from: https://www-tandfonline-com.ezproxy.library.ubc.ca/doi/full/10.1080/02786826.2020.1749229
  8. World Health Organization: WHO. “Modes of Transmission of Virus Causing COVID-19: Implications for IPC Precaution Recommendations.” Who.Int, World Health Organization: WHO, 29 Mar. 2020, www.who.int/news-room/commentaries/detail/modes-of-transmission-of-virus-causing-covid-19-implications-for-ipc-precaution-recommendations. Accessed 17 Apr. 2020.
  9. Liu L, Wei J, Li Y, Ooi A. Evaporation and dispersion of respiratory droplets from coughing. Indoor Air [Internet]. 2016 Mar 23 [cited 2020 Apr 18];27(1):179–90. Available from: https://onlinelibrary-wiley-com.ezproxy.library.ubc.ca/doi/epdf/10.1111/ina.12297
  10. Morawska L. Droplet fate in indoor environments, or can we prevent the spread of infection? Indoor Air [Internet]. 2006 Oct [cited 2020 Apr 18];16(5):335–47. Available from: https://onlinelibrary.wiley.com/doi/full/10.1111/j.1600-0668.2006.00432.x
  11. Chen W, Zhang N, Wei J, Yen H-L, Li Y. Short-range airborne route dominates exposure of respiratory infection during close contact. Building and Environment [Internet]. 2020 Jun [cited 2020 Apr 18];176:106859. Available from: https://www-sciencedirect-com.ezproxy.library.ubc.ca/science/article/pii/S0360132320302183

Disclaimer

The above is intended to serve as a rapidly-created, accessible source of information curated by medical students and healthcare professionals. It is for educational purposes only and is not a complete reference resource. It is not professional medical advice, and is not a substitute for the discretion, judgment, and duties of healthcare professionals. You are solely responsible for evaluating the information above.