Crowding and the epidemic intensity of COVID-19 transmission

Benjamin Rader; Samuel Scarpino; Anjalika Nande; Alison Hill; Benjamin Dalziel; Robert Reiner; David Pigott; Bernardo Gutierrez; Munik Shrestha; John Brownstein; Marcia Castro; Huaiyu Tian; Bryan Grenfell; Oliver Pybus; Jessica Metcalf; Moritz U.G. Kraemer

higher-peaked outbreaks. To capture more realistic contact patterns, we created hierarchically-structured 169 populations 28 where individuals had high rates of contact within their households (households are defined 170 broadly and could represent care homes, hospitals, prisons, etc.), lower rates with individuals from other 171 households but within the same "neighborhoods", and relatively rare contact with other individuals in the 172 same prefecture (Figure 4A) . Assumptions are consistent with reports that the majority of onward 173 transmission occurred in households 2,29 . We assumed spread between prefectures was negligible once an 174 outbreak started. In this scenario, "sparse" prefectures often had more intense, short-term outbreaks that 175 were isolated to certain neighborhoods, while "crowded" prefectures could have drawn-out, low intensity 176 outbreaks that jumped between the more highly-connected "neighborhoods" (Figures 4B and C) . These 177 outbreaks had larger final size than those in less-crowded areas ( Figure 4C ) which likely is related to 178 large overdispersion in the reproduction number of COVID-19 30,31 where local outbreaks can reach their 179 full potential due to the availability of contacts. We also considered outbreak dynamics in sparse and 180 crowded prefectures under strong social distancing measures, which is likely to be the scenario occurring 181