Mapping the Burden of COVID-19 in the United States

Ian F Miller; Alexander D Becker; Bryan T Grenfell; C. Jessica E Metcalf

We developed a modified age-stratified Susceptible-Exposed-Infected-Recovered (SEIR) epidemic model (based on the model of Davies et al. 7 ) to project the number of cases originating in each U.S. country. In this model, susceptible individuals (S) become infected in a density dependent fashion and enter the exposed (E) class, before eventually becoming either asymptomatically infected (IA) or 'pre-clinically' infected (e.g. symptomatic but not yet clinically presenting) (IP). The fraction of individuals who become pre-clinically infected rather than asymptomatically infected increases with age according to published estimates 7 . Pre-symptomatic individuals eventually become symptomatic (IC). Asymptomatic and symptomatic individuals eventually recover with immunity to classes RA and RS respectively. All individuals in the infected classes (IA, IP, IC) are infectious; however, our model assumes that the relative infectiousness of asymptomatic individuals is reduced by a factor ba, and the relative infectiousness of clinical individuals is reduced by a factor bC to account for the effects of case isolation and quarantine.