> In addition to [reproduction number] R, scientists use a value called the dispersion factor (k), which describes how much a disease clusters. The lower k is,

> In addition to [reproduction number] R, scientists use a value called the dispersion factor (k), which describes how much a disease clusters. The lower k is, the more transmission comes from a small number of people. SARS (2005): k ~= 0.16 MERS (2012): k ~= 0.25 Spanish Flu (1918): k ~= 1 Estimates of k for Covid-19 vary: - An earlier study in Hong Kong said k was slightly higher than for SARS or MERS. - Recent study [1] based on data outside China puts k ~= 0.1, which would mean 10% cases lead to 80% of the spread. - Lower k would explain why the virus didn't spread all around the world immediately, and why many infection clusters seem to clear on their own. Large infection clusters are thought to appear due to aerosol transmission, in particular indoors, where the risk is 19 times higher according to another study [2]. 1. https://wellcomeopenresearch.org/articles/5-67 https://wellcomeopenresearch.org/articles/5-67 2. https://www.medrxiv.org/content/10.1101/2020.02.28.20029272v2 https://www.medrxiv.org/content/10.1101/2020.02.28.20029272v...