Calculating Risks of Transmission

 

There has been much debate about whether SARS-CoV-2 (the virus producing the Covid-19 pandemic) is, to a meaningful extent, transmitted by the aerosol route as people breathe, talk or sing as well as by coughs and sneezes. There has also been much discussion about the reliability of mathematical models to determine risk of infection. The National Geographic has usefully detailed the growing evidence for viral transmission in micro-droplets and high-lighted some very interesting modelling of risk carried out by Professor Jose-Luis Jimenez at the University of Boulder, Colorado (https://www.nationalgeographic.com/science/2020/08/how-to-measure-risk-airborne-coronavirus-your-office-classroom-bus-ride-cvd/?cmpid=org=ngp::mc=crm-email::src=ngp::cmp=editorial::add=SpecialEdition_20200814&rid=FE3832908123A963FB7A467BD75D4FAD). Jimenez (a supporter of aerosol transmission) has calculated the risks of transmission with and without an air-tight N95 mask (one that blocks 95% of particles) to 20 exposures of a range of everyday scenarios. The scenarios included an indoor gathering; working in an energy-efficient office; participating in a classroom lecture; performing strenuous indoor activity; riding on the subway (underground) and travelling by bus.Variables, such as operating in a low (0.03% ) or a high (3%) infection area, as well as changing the amount of personal space that individual had were also looked at. I don't think that one can be absolutely confident about the percentage risk of particular activities (behaviours in classrooms or on public transport can be quite variable). A number of assumptions also had to be made (e.g. no level of immunity in the populations, fixed environmental conditions et cetera) before the calculations could be made. There are, however, a number of predictable 'take home' messages namely that a) a well-fitting mask does greatly lessen the risk of infection, b) activities vary in terms of their riskiness (reflecting the numbers of people involved?); c) transmission is much more likely in high infection areas and d) crowding increases the risk of transmission.