Household exposure to SARS-CoV-2 and association with COVID-19 severity: a Danish nationwide cohort study

Broccia, Marcella M; de Knegt, Victoria Elizabeth V E,; Mills, Elisabeth Helen Anna E H A,; Møller, Amalie Lykkemark A L; Gnesin, Filip F; Fischer, Thea K T K,; Zylyftari, Nertila N; Blomberg, Stig Nikolaj S N,; Andersen, Mikkel Porsborg M P,; Schou, Morten M; Fosbøl, Emil E; Kragholm, Kristian K; Christensen, Helle Collatz H C; Polcwiartek, Laura Bech L B,; Phelps, Matthew M; Køber, Lars L; Torp-Pedersen, Christian C

BACKGROUND: Households are high-risk settings for the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Severity of Coronavirus disease 2019 (COVID-19) is likely associated with the infectious dose of SARS-CoV-2 exposure. We therefore aimed to assess the association between SARS-CoV-2 exposure within households and COVID-19 severity. METHODS: We performed a Danish nationwide register-based cohort study including laboratory-confirmed SARS-CoV-2 infected individuals from 22 February to 6 October 2020. Household exposure to SARS-CoV-2 was defined as having one individual tested positive for SARS-CoV-2 within the household. Cox proportional-hazards models were used to estimate the association between ‘critical COVID-19’ within and between households with and without secondary cases. RESULTS: From 15,063 multi-person households, 19,773 SARS-CoV-2 positive individuals were included; 11,632 were categorized as index cases without any secondary household cases, 3,431 as index cases with secondary cases, i.e. 22.8% of multi-person households, and 4,710 as secondary cases. ‘Critical COVID-19’ occurred in 2.9 % of index cases living with no secondary cases, 4.9 % of index cases with secondary cases, and 1.3 % of secondary cases. The adjusted hazard ratio for ‘critical COVID-19’ among index cases versus secondary cases within the same household was 2.50 (95%CI=1.88-3.34), 2.27 (95%CI=1.77-2.93) for index cases in households with no secondary cases versus secondary cases, and 1.1 (95%CI=0.93-1.30) for index cases with secondary cases versus index cases without secondary cases. CONCLUSION: We found no increased hazard ratio of ‘critical COVID-19’ among household members of infected SARS-CoV-2 index cases.