Characteristics Associated With Household Transmission of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) in Ontario, Canada: A Cohort Study

Abstract

Background: Within-household transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection has been identified as one of the main sources of spread of coronavirus disease 2019 (COVID-19) after lockdown restrictions and self-isolation guidelines are implemented. Secondary attack rates among household contacts are estimated to be 5-10 times higher than among non-household contacts, but it is unclear which individuals are more prone to transmit infection within their households.

Methods: Using address matching, a cohort was assembled of all individuals with laboratory-confirmed COVID-19 residing in private households in Ontario, Canada. Descriptive analyses were performed to compare characteristics of cases in households that experienced secondary transmission versus those that did not. Logistic regression models were fit to determine index case characteristics and neighborhood characteristics associated with transmission.

Results: Between January and July 2020, there were 26 714 individuals with COVID-19 residing in 21 226 households. Longer testing delays (≥5 vs 0 days; odds ratio [OR], 3.02; 95% confidence interval [CI], 2.53-3.60) and male gender (OR, 1.28; 95% CI, 1.18-1.38) were associated with greater odds of household secondary transmission, while being a healthcare worker (OR, .56; 95% CI, .50-.62) was associated with lower odds of transmission. Neighborhoods with larger average family size and a higher proportion of households with multiple persons per room were also associated with greater odds of transmission.

Conclusions: It is important for individuals to get tested for SARS-CoV-2 infection as soon as symptoms appear, and to isolate away from household contacts; this is particularly important in neighborhoods with large family sizes and/or crowded households.

Keywords: COVID-19; SARS-CoV-2; household; transmission.

Figure 1.

Flow diagram of study cohort. Abbreviation: COVID-19, coronavirus disease 2019.

Figure 2.

Adjusted ORs and 95% CIs for the associations between index case delay metrics and odds of household transmission. A total of 269 cases were excluded from the testing delay models that had COVID-19 symptoms flagged in provincial reportable disease systems but were missing symptom-onset date. Cases with no symptoms were defined as cases who were missing symptom-onset date (thus, specimen collection date was used) and did not have any COVID-19 symptoms flagged in provincial reportable disease systems. Cases with a testing delay of <0 days were those who were tested prior to the onset of their symptoms. Horizontal dashed line at OR = 1 indicates no association. Abbreviations: CI, confidence interval; COVID-19, coronavirus disease 2019; No symp, no symptoms; OR, odds ratio.