SARS-CoV-2 transmission in a highly vulnerable population of Brazil: a household cohort study

Background Household transmission studies seek to understand the transmission dynamics of a pathogen by estimating the risk of infection from household contacts and community exposures. We estimated within/extrahousehold SARS-CoV-2 infection risk and associated factors in a household cohort study in one of the most vulnerable neighbourhoods in Rio de Janeiro city. Methods Individuals >= 1 years-old with suspected or confirmed fi rmed COVID-19 in the past 30 days (index cases) and household members aged >= 1 year were enrolled and followed at 14 and 28 days (study period November/2020-December/2021). - December/2021). RTPCR testing, COVID-19 symptoms, and SARS-CoV-2 serologies were ascertained in all visits. Chain binomial household transmission models were fi tted using data from 2024 individuals (593 households). Findings Extra-household infection risk was 74.2% (95% credible interval [CrI] 70.3-77.8), - 77.8), while within-household infection risk was 11.4% (95% CrI 5.7-17.2). - 17.2). Participants reporting having received two doses of a COVID-19 vaccine had lower extra-household (68.9%, 95% CrI 57.3-77.6) - 77.6) and within-household (4.1%, 95% CrI 0.4-16.6) - 16.6) infection risk. Within-household infection risk was higher among participants aged 10-19 - 19 years, from overcrowded households, and with low family income. Contrastingly, extra-household infection risk was higher among participants aged 20-29 - 29 years, unemployed, and public transportation users. Interpretation Our study provides important insights into COVID-19 household/community transmission in a vulnerable population that resided in overcrowded households and who struggled to adhere to lockdown policies and social distancing measures. The high extra-household infection risk highlights the extreme social vulnerability of this population. Prioritising vaccination of the most socially vulnerable could protect these individuals and reduce widespread community transmission. Funding Funda & ccedil;& atilde;o Oswaldo Cruz, CNPq, FAPERJ, Royal Society, Instituto Serrapilheira, FAPESP.