Structural risk factors for SARS-CoV-2 infection in an urban slum: Age, gender and income (preprint)

Background The structural environment of urban slums, including physical, demographic and socioeconomic attributes, renders inhabitants more vulnerable to SARS-CoV-2 infection. Yet, little is known about the specific determinants that contribute to high transmission within these communities. Methods and findings We performed a serosurvey of an established cohort of 2,035 urban slum residents from the city of Salvador, Brazil between November 2020 and February 2021, following the first COVID-19 pandemic wave in the country. We identified high SARS-CoV-2 seroprevalence (46.4%, 95% confidence interval [CI] 44.3-48.6%), particularly among female residents (48.7% [95% CI 45.9-51.6%] vs. 43.2% [95% CI 39.8-46.6%] among male residents), and among children (56.5% [95% CI 52.3-60.5%] vs. 42.4% [95% CI 39.9-45.0%] among adults). In multivariable models that accounted for household-level clustering, the odds ratio for SARS-CoV-2 seropositivity among children was 1.96 (95% CI 1.42-2.72) compared to adults aged 30-44 years. Adults residing in households with children were more likely to be seropositive; this effect was particularly prominent among individuals with age 30-44 and 60 years or more. Women living below the poverty threshold (daily per capita household income

Reliable estimation of SARS-CoV-2 anti-spike protein IgG titers from single dilution optical density values in serologic surveys (preprint)

Background: As the COVID-19 pandemic evolves, there is a need for reliable and scalable seroepidemiology methods to estimate incidence, monitor the dynamics of population-level immunity, and guide mitigation and immunization policies. Our aim was to evaluate the reliability of normalized ELISA optical density (nOD) at a single dilution as a predictor of SARS-CoV-2 immunoglobulin titers derived from serial dilutions. Methods: We conducted serial serological surveys of a community-based cohort from the city of Salvador, Brazil after two sequential COVID-19 epidemic waves. Anti-SARS-CoV-2 spike protein immunoglobulin G (anti-S IgG) ELISA (Euroimmun AG) was performed with serial 3-fold dilutions of sera from 54 of the 1101 cohort participants. We estimated interpolated ELISA titers, used parametric models to fit the relationship between nOD at a single 1:100 dilution and interpolated titers, and assessed the correlation between changes in nOD and changes in titers. Results: The relationship between nOD at a single 1:100 dilution and interpolated titers fit a log-log curve, with a residual standard error of 0.304. We derived a conversion table of nOD to interpolated titer values. Additionally, there was a high correlation between changes in nOD and changes in interpolated titers between paired serial samples (r = 0.836, {rho} = 0.873). Changes in nOD reliably predicted increases and decreases in titers, with 98.1% agreement ({kappa} = 95.9%). Conclusion: Single nOD measurements can reliably estimate SARS-CoV-2 antibody titers, significantly reducing time, labor, and resource needs when conducting large-scale serological surveys to ascertain population-level changes in exposure and immunity.