Los Angeles County SARS-CoV-2 Epidemic: Critical Role of Multi-generational Intra-household Transmission

We studied the incidence of confirmed COVID-19 cases diagnosed during February 24, 2020 -January 10, 2021 in approximately 300 communities making up Los Angeles County, the largest county by population in the United States. The surge in case incidence observed from October 19 onward, accounting for two-thirds of all confirmed cases, was concentrated in communities with a high prevalence of multi-generational households, as gauged by data from the American Community Survey. This indicator (abbreviated MULTI) was a more important predictor of the surge in incidence than the prevalence of households with low income or with at least one high-risk worker. Serial mapping of the epidemic revealed radial expansion from an initial focus in relatively affluent communities, followed by concentration in high-MULTI communities. This observation was supported by estimates from a spatial adaptation of the SIR model, which yielded a reproductive number of 2.7 for the initial outbreak during February 24 -March 30. With the subsequent flattening of the epidemic curve after the imposition of emergency stay-at-home orders, the global reproductive number fell to 1.0, but with wide local dispersion ranging from 0.6 in low-MULTI communities up to 1.5 in high-MULTI communities. The July 13 state-ordered reversal of the county’s prior decisions to reopen retail stores, indoor dining, hair salons, gyms and bars had a larger negative impact on social mobility in high-MULTI communities, as gauged by data from SafeGraph on smartphone visits to fast-food restaurants. After falling to a low of 0.6, the reproductive number rebounded to 1.4 during the final surge. By the end of the 46-week observation period, the estimated cumulative incidence of COVID-19, adjusted for underascertainment of both asymptomatic and symptomatic cases, ranged from under 10 percent in low-MULTI communities to over 30 percent in high-MULTI communities.