Understanding the Los Angeles County Coronavirus Epidemic: The Critical Role of Intrahousehold Transmission

Harris, J. E.

We tracked the course of the COVID-19 epidemic among the approximately 300 communities comprising Los Angeles County. The epidemic, we found, had three distinct phases. During Phase I, from early March through about April 4, initial seeding of infection in relatively affluent areas was followed by radial geographic extension to adjoining communities. During Phase II, which continued to about July 11, COVID-19 cases continued to rise at a slower rate, and became increasingly concentrated in four geographic foci of infection across the county. This phase saw changes in two indicators of social mobility: the proportion of location-tracked smartphones staying at home, and the number of smartphones visiting a gym. For both indicators, those communities with a larger reduction in social mobility during April reported fewer new COVID-19 cases in May. During Phase III, COVID-19 incidence only gradually declined, remaining as high as the incidence seen at the end of Phase I. Across communities, the prevalence of households at high risk for intergenerational transmission was strongly correlated with the persistence of continued COVID-19 propagation. This association was even stronger in those communities with a higher rate of gym attendance in Phase II. The map of the prevalence of at-risk households in Los Angeles County coincided strikingly with the map of cumulative COVID-19 incidence. These findings, taken together, support the critical role of household structure in the persistent propagation of COVID-19 infections in Los Angeles County. Public health policy needs to be reoriented from a focus on protecting the individual to a focus on protecting the household.