Household transmission of COVID-19 in 2020 in New South Wales, Australia.

Abstract

Households are high-risk settings for the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This study examines factors associated with transmission among cases diagnosed with coronavirus disease 2019 (COVID-19) and their household contacts, in New South Wales (NSW), Australia, during July-October 2020. A register of all laboratory-confirmed COVID-19 cases was used to extract demographic and clinical information for cases and household contacts. Secondary attack rates (SARs) among household members were calculated and generalised estimating equations were used to estimate risks of transmission in relation to various characteristics of the primary case and the household contacts. In total, 229 households were included; they consisted of 229 primary cases and 659 close contacts. The overall household SAR was 22.5% (148/659). After adjusting for symptoms, age and sex of primary case, spouse status of household contacts and household size, the odds of secondary transmission were lower in primary cases who were asymptomatic at diagnosis than in symptomatic cases (odds ratio, OR: 0.13; 95% confidence interval (95% CI): 0.04-0.48); and higher in primary cases aged 60 years and over than in those aged 19-39 years (OR: 3.45; 95% CI: 1.53- 7.75). Being a spouse of the primary case was also associated with increased transmission compared to non-spouses (OR: 1.93; 95% CI: 1.24-3.02). After adjustments, there was no significant effect on transmission of the primary case's sex, or of the number of people in the household. This study documents demographic and clinical characteristics that increase transmission rates in households in the period prior to the introduction of SARS-CoV-2 variants. These data can be used as a baseline from which to compare household transmission in outbreaks dominated by new variants.