Identification of key COVID-19 infection contexts over time and the importance of within household infections

Objectives: Infectious disease outbreaks, most recently the COVID-19 pandemic, have broad societal impacts and may require the introduction of costly non-pharmaceutical interventions (NPIs). To minimize these burdens, the identification of infection contexts is paramount: knowing where infections occur allows the determination of high-risk environments, the identification of outbreak sources, and finally the introduction of targeted, and thus cost-efficient NPIs. Methods: Starting in February 2020, the Cologne public health department interviewed all PCR-confirmed COVID-19 cases residing in Cologne regarding their probable infection context. The collected individual-level data was entered into an in-house-developed software. From these interviews, aggregated case numbers for the different infection contexts are available at the daily level. The manuscript provides an explorative analysis of the aggregated data. Results: During 2021, within-one’s-own-household was the dominant known infection context in Cologne. Following within-own-household-infections, the contributions differed over time, with notable contributions of shared accommodations (specifically nursing homes) early in the year, large contributions of disease import during the summer of 2021, and increasing contributions of family (different household) and leisure miscellaneous over the course of 2021. Conclusion: The analysis reveals that the combination of state-mandated NPIs and voluntary behavior adaptations strongly influenced the regional spreading dynamics and managed to limit infections in critical contexts, such as nursing homes.