Risk of SARS-CoV-2 Transmissions
This latest publication in BMJ, published 25 August 2020, by the University of Oxford has attempted to qualify the risk of SARS-CoV-2 transmission from asymptomatic people in different settings.
This guide shows how transmission risk may vary with setting, occupancy level, contact time, and face coverings. This graph is intended to be an initial quick risk assessment.
Current rules on safe physical distancing are based on outdated science as new evidence suggests that SARS-CoV-2 can travel further distances through forceful activities such as coughing or shouting/singing/panting. In addition, distribution of viral particles is a complex entity affected by numerous factors, including air flow, ventilation etc.
Rules on distancing should reflect the multiple factors that affect risk, including ventilation, occupancy, and exposure time. Their recommendations based on this guide are:
1- In the highest risk situations (indoor environments with poor ventilation, high levels of occupancy, prolonged contact time, and no/limited face coverings) physical distancing beyond 6ft/2m and minimising occupancy time should be considered. In short, distancing may not be effective indoors if there is poor ventilation and high contact time.
2- In low risk situations, less stringent distancing is likely to be adequate. Therefore, there is room for more nuanced guidelines depending on situation.
– People with symptoms tend to have high viral load and more frequent violent respiratory exhalations. However, these were not included in the guide as the assumption is that those who are symptomatic will be isolating.
– Face coverings were described as non high-grade respirators.
Caveats: Per the author, Prof Greenhalgh, due to multiple variables in each situation, this is meant to be an indicative table and not expected to be predictive or quantitative. Hence, they do not include strict definitions of ‘prolonged time’ or ‘low vs high occupancy’.
The levels of risk are relative and not absolute. What this means is that the risk varies based on multiple variables and also changes based on additional factors such as viral shedding load of an infected person, indoor airflow patterns, and the exact location of an individual in relation to the infected person. All of these factors can increase or decrease the absolute risk.