Social determinants of adult COVID-19 vaccine acceptance and uptake in a Brazilian urban informal community: a longitudinal time-to-event study

Residents of informal urban settlements have a high risk of COVID-19 exposure and have less access to medical care, making vaccine-driven prevention critical in this vulnerable population. Despite robust vaccination campaigns in Brazil, vaccine uptake and timing continue to be influenced by social factors and contribute to health disparities. To address this, we conducted a sequential survey in a cohort of 717 adults in an urban favela in Salvador, Brazil where participants were interviewed in 2020, before vaccines were rolled out, and in 2022, after primary and booster dose distribution. We collected data on demographics, social characteristics, and COVID-19 vaccination status and intent. Primary series uptake was high (91.10% for 1st dose and 94.74% for 2nd dose among eligible); however, booster uptake was lower (63.51% of eligible population) at the time of the second interview, suggesting a decreasing interest in vaccination. To account for both vaccine refusal and delays, we conducted a Cox time-to-event analysis of dose uptake using sequential independent outcomes. Exposure times were determined by dose eligibility date to account for age and comorbidities. Intent to vaccinate in 2020 (hazard ratio [HR]: 1.54, CI: [1.05, 1.98]) and age (HR: 1.27, CI: [1.01, 2.08]) were associated with higher vaccination rates for the 1st dose. Males were less likely to receive the 1st dose (HR: 0.61, CI: [0.35, 0.83]), and, compared to catholics, 2nd dose uptake was lower for those identifying with Pentecostalism (HR: 0.49, CI: [0.37, 0.66]) and without a religion (HR: 0.49, CI: [0.37, 0.66]), with the latter association disappearing after controlling by age. Risk perception was associated with 2nd dose uptake (HR: 1.15, CI: [1.08, 1.26]). The role of sex and religion in vaccination behavior highlights the need for targeted outreach and interfacing with local organizations. The data offers lessons to build a long-term COVID-19 vaccination strategy beyond availability.