Household transmission dynamics of COVID-19 among residents of Delhi, India: a prospective case-ascertained study

•Household settings give a clearer picture of infectious disease transmission dynamics.•Four dimensions of risk factors for understanding secondary infection are proposed.•A high secondary attack rate highlights the need for COVID-appropriate behaviours.•A targeted approach could be adopted in limiting disease among household contacts.•Our four-dimensional approach to understanding household transmission is relevant. The aim of this study was to observe the secondary infection rate and transmission dynamics of COVID-19 among household contacts, and their associations with various factors across four dimensions of interaction. This was a case-ascertained study among unvaccinated household contacts of a laboratory-confirmed COVID-19 case in New Delhi between December 2020 and July 2021. For this study, 99 index cases and their 316 household contacts were interviewed and sampled (blood and oro-nasal swab) on days 1, 7, 14, and 28. The secondary infection rate among unvaccinated household contacts was 44.6% (95% confidence interval (CI) 39.1–50.1). The predictors of secondary infection among individual contact levels were: being female (odds ratio (OR) 2.13), increasing age (OR 1.01), symptoms at baseline (OR 3.39), and symptoms during follow-up (OR 3.18). Among index cases, age of the primary case (OR 1.03) and symptoms during follow-up (OR 6.29) were significantly associated with secondary infection. Among household-level and contact patterns, having more rooms (OR 4.44) and taking care of the index case (OR 2.02) were significantly associated with secondary infection. A high secondary infection rate highlights the need to adopt strict measures and advocate COVID-19-appropriate behaviors. A targeted approach for higher-risk household contacts would efficiently limit infections among susceptible contacts. [Display omitted]