COVID-19 outbreaks in nursing homes in Los Angeles County, March 2020-April 2022.

Introduction: Nursing home (NH) residents have an elevated risk of coronavirus disease 2019 (COVID-19) infection and severe outcomes. However, literature regarding outbreak outcomes at the facility level is limited.

Methods: NH outbreaks beginning between March 1, 2020, and February 22, 2022, at facilities under Los Angeles County jurisdiction were assigned to 1 of 6 time periods defined by dominant variants, surges in community transmission, and vaccination levels. Outbreaks were defined as 1 or more NH resident cases with lab-confirmed COVID-19 infection and no periods of 14 or more days between successive COVID-19 resident cases. Outbreak size and duration were the number of NH resident cases and the number of days between the index case(s) and 14 days after the last resident case(s). Rates of severe outcomes were measured per 100 licensed beds among all outbreaks per time period.

Results: 44,279 cases were analyzed from 1,587 outbreaks. Median outbreak duration peaked during the first winter surge (39 days; time period 3); median outbreak size per 100 licensed beds peaked in time period 6 (17), after widespread vaccination - during the second winter surge. Hospitalizations and deaths per 100 resident cases fell from 31 and 24 prior to widespread vaccination to 11 and 7, respectively, after.

Conclusions: NH COVID-19 outbreaks may have been affected by vaccine uptake and community transmission levels. Because outbreak size and duration peaked during peak community transmission but severe outcome rates did not, the latter may be preferable to outbreak size and duration as outbreak metrics.